[SCM] Git init command

[svenefftinge]

In case the workspace isn't a git repository (or other SCM repository) there is a weird message in the SCM widget. The SCM provider's should be asked if they can init a workspace and in case they do show a button to the user.

Leaving all the abstraction stuff aside, there should be a button to init a git repository :)

[akosyakov]

That how it looks in VS Code: https://code.visualstudio.com/docs/editor/versioncontrol#_initialize-a-repository

[svenefftinge]

We discussed to do the following:

• register a command and toolbar (+) in git
• fix the message to 'No repository found' (align rest with screenshot above)
• make all other commands invisible when no repository found.