[plug-in] Implement 'views' and 'viewsContainers' contribution points and Tree API

[evidolob]

VS Code has views contributions.
Also registerTreeDataProvider and createTreeView methods should be implemented.

Sub task:

• Implement 'viewsContainers' contribution point
• Implement 'views' contribution point
• Implement registerTreeDataProvider and createTreeView methods

[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"
                }
            ]
        }