[plug-in] Implement 'viewsContainers' contribution point

[evidolob]

Configuration example:

"contributes": {
        "viewsContainers": {
            "activitybar": [
                {
                    "id": "files",
                    "title": "Files",
                    "icon": "resources/files.svg"
                }
            ]
        }
}

VS Code sample:

Also add possibility to point in what part of Theia layout viewsContainers can be added:

"contributes": {
        "viewsContainers": {
            "activitybar": [
                {
                    "id": "files",
                    "title": "Files",
                    "icon": "resources/files.svg",
                    "position": "left" | "bottom" | "right" 
                }
            ]
        }
  }

Implementation should use current Theia layout system:

VSCode documentation

[vitaliy-guliy]

I suppose it's better to give up using "position": "left" | "bottom" | "right" and use named targets for the view containers.
Let's use names "left" | "activitybar" for the left part, "right" for the right and "bottom" for the bottom.
We must support "activitybar" target to have some support of VSCode plugins.

"contributes": {
        "viewsContainers": {
            "activitybar": [
                {
                    "id": "files",
                    "title": "Files",
                    "icon": "resources/files.svg"
                }
            ],
            "right": [
                {
                    "id": "pull-request",
                    "title": "Pull Request",
                    "icon": "resources/pull.svg"
                }
            ]
        }
  }

[vitaliy-guliy]

To test views and views containers add following section to package.json of plugin in contributes section

        "viewsContainers": {
            "activitybar": [
                {
                    "id": "debug",
                    "title": "Debug",
                    "icon": "resources/debug.svg"
                }
            ],
            "left": [
                {
                    "id": "package-explorer",
                    "title": "Package Explorer",
                    "icon": "resources/package-explorer.svg"
                }
            ],
            "right": [
                {
                    "id": "comments",
                    "title": "Comments",
                    "icon": "resources/comments.svg"
                }
            ],
            "bottom": [
                {
                    "id": "remote",
                    "title": "Remote",
                    "icon": "resources/remote.svg"
                }
            ]
        },
        "views": {
            "package-explorer": [
                {
                    "id": "package-dependencies",
                    "name": "Package Dependencies"
                },
                {
                    "id": "package-outline",
                    "name": "Package Outline"
                }
            ],
            "comments": [
                {
                    "id": "ann-comments",
                    "name": "Anna's Comments"
                },
                {
                    "id": "sun-comments",
                    "name": "Sun's Comments"
                },
                {
                    "id": "yevhen-comments",
                    "name": "Yevhen's Comments"
                }
            ],
            "remote": [
                {
                    "id": "remote-terminal",
                    "name": "Remote terminal"
                }
            ]
        }