[1316][1339] Enhance the view model

Now there are two separated concepts:
 * The node style description concept that tells how the node is
represented
 * The new concept of LayoutStrategyDescription that tells how the
children are layouted.

* Add new Styles
 - RectangularNodeStyleDescription
 - ImageNodeStyleDescription
 - IconLabelNodeStyleDescription
* Add a "childrenLayoutStrategy" feature to NodeDescription. There are
two implementations
 - FreeFormLayoutStrategyDescription
 - ListLayoutStrategyDescription

Bug: #1316
Bug: #1339
Signed-off-by: Laurent Fasani <laurent.fasani@obeo.fr>

packages/view/backend/sirius-components-view-edit/src/main/java/org/eclipse/sirius/components/view/provider/DiagramDescriptionItemProvider.java

@@ -184,7 +184,8 @@ protected void collectNewChildDescriptors(Collection<Object> newChildDescriptors
 
         NodeDescription nodeChild = ViewFactory.eINSTANCE.createNodeDescription();
         nodeChild.setName("Node"); //$NON-NLS-1$
-        nodeChild.setStyle(ViewFactory.eINSTANCE.createNodeStyle());
+        nodeChild.setStyle(ViewFactory.eINSTANCE.createRectangularNodeStyleDescription());
+        nodeChild.setChildrenLayoutStrategy(ViewFactory.eINSTANCE.createFreeFormLayoutStrategyDescription());
         newChildDescriptors.add(this.createChildParameter(ViewPackage.Literals.DIAGRAM_DESCRIPTION__NODE_DESCRIPTIONS, nodeChild));
 
         EdgeDescription edgeChild = ViewFactory.eINSTANCE.createEdgeDescription();

packages/view/backend/sirius-components-view-edit/src/main/java/org/eclipse/sirius/components/view/provider/FreeFormLayoutStrategyDescriptionItemProvider.java

@@ -0,0 +1,124 @@
+/**
+ * Copyright (c) 2021, 2022 Obeo.
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *      Obeo - initial API and implementation
+ */
+package org.eclipse.sirius.components.view.provider;
+
+import java.util.Collection;
+import java.util.List;
+
+import org.eclipse.emf.common.notify.AdapterFactory;
+import org.eclipse.emf.common.notify.Notification;
+import org.eclipse.emf.common.util.ResourceLocator;
+import org.eclipse.emf.edit.provider.IEditingDomainItemProvider;
+import org.eclipse.emf.edit.provider.IItemLabelProvider;
+import org.eclipse.emf.edit.provider.IItemPropertyDescriptor;
+import org.eclipse.emf.edit.provider.IItemPropertySource;
+import org.eclipse.emf.edit.provider.IStructuredItemContentProvider;
+import org.eclipse.emf.edit.provider.ITreeItemContentProvider;
+import org.eclipse.emf.edit.provider.ItemProviderAdapter;
+
+/**
+ * This is the item provider adapter for a {@link org.eclipse.sirius.components.view.FreeFormLayoutStrategyDescription}
+ * object. <!-- begin-user-doc --> <!-- end-user-doc -->
+ *
+ * @generated
+ */
+public class FreeFormLayoutStrategyDescriptionItemProvider extends ItemProviderAdapter
+        implements IEditingDomainItemProvider, IStructuredItemContentProvider, ITreeItemContentProvider, IItemLabelProvider, IItemPropertySource {
+    /**
+     * This constructs an instance from a factory and a notifier. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    public FreeFormLayoutStrategyDescriptionItemProvider(AdapterFactory adapterFactory) {
+        super(adapterFactory);
+    }
+
+    /**
+     * This returns the property descriptors for the adapted class. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public List<IItemPropertyDescriptor> getPropertyDescriptors(Object object) {
+        if (this.itemPropertyDescriptors == null) {
+            super.getPropertyDescriptors(object);
+
+        }
+        return this.itemPropertyDescriptors;
+    }
+
+    /**
+     * This returns FreeFormLayoutStrategyDescription.gif. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public Object getImage(Object object) {
+        return this.overlayImage(object, this.getResourceLocator().getImage("full/obj16/FreeFormLayoutStrategyDescription")); //$NON-NLS-1$
+    }
+
+    /**
+     * <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    protected boolean shouldComposeCreationImage() {
+        return true;
+    }
+
+    /**
+     * This returns the label text for the adapted class. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public String getText(Object object) {
+        return this.getString("_UI_FreeFormLayoutStrategyDescription_type"); //$NON-NLS-1$
+    }
+
+    /**
+     * This handles model notifications by calling {@link #updateChildren} to update any cached children and by creating
+     * a viewer notification, which it passes to {@link #fireNotifyChanged}. <!-- begin-user-doc --> <!-- end-user-doc
+     * -->
+     *
+     * @generated
+     */
+    @Override
+    public void notifyChanged(Notification notification) {
+        this.updateChildren(notification);
+        super.notifyChanged(notification);
+    }
+
+    /**
+     * This adds {@link org.eclipse.emf.edit.command.CommandParameter}s describing the children that can be created
+     * under this object. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    protected void collectNewChildDescriptors(Collection<Object> newChildDescriptors, Object object) {
+        super.collectNewChildDescriptors(newChildDescriptors, object);
+    }
+
+    /**
+     * Return the resource locator for this item provider's resources. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public ResourceLocator getResourceLocator() {
+        return ViewEditPlugin.INSTANCE;
+    }
+
+}