Offer richer UI API to VS Code extensions

[tsmaeder]

Feature Description:

VS Code severely limits the things plugins can do, we might become a more attractive alternative for the "fork VS Code" crowd if we offered richer UI in a separate theia API namespace like

1. Global selection
   If we had the concept of a "global selection" or "navigator selection", it would allow us to have views that show "details" for what's selected somewhere else.
2. Wizards
   We could allow plugins to define wizards and dialogs.

Before we could do this, though, we would have to clean up the plugin engine to

• separate the theia and vscode namespaces
• allow for using theia and vscode namespaces in parallel.