TypeScript conversion for RectangularPushButton/PushButtonModel (and …

…inherited types), Slider/SliderTrack (and associated types), NumberControl/ArrowButton/NumberDisplay, see phetsims/scenery-phet#726

js/Property.ts

@@ -56,7 +56,13 @@ type PropertyOptions<T> = Partial<PropertyDefinedOptions> & {
 } & PhetioObjectOptions;
 
 // a Property (can't be a TinyProperty) with all of the value-mutation removed.
-export type ReadOnlyProperty<T> = Omit<Property<T>, 'set' | 'reset'> & { readonly value: T; };
+export interface ReadOnlyProperty<T> extends Property<T> {
+  readonly value: T;
+
+  // Any solution with Omit<Property<T>, 'set' | 'reset'> was failing, as it wasn't assignable to PhetioObject
+  set: unknown & any;
+  reset: unknown & any;
+}
 
 class Property<T> extends PhetioObject implements IProperty<T> {