Add git module

This is an initial implementation of a git module.
When Frescobaldi is run from Git this module is loaded and a `git.app` object instantiated.
This represents Frescobaldi's own Git repository.

A few initial functions have been implemented.
The visible result is the printing of the branch/remote in Frescobaldi's title bar.

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The next thing I would like to do is add a `Git` menu offering:
- look for updates (simple mode: only for `master` in the upstream repo)  
  (optionally do that automatically on program startup)
- create menu entries for each local branch and  submenus for each registered remote.  
  Clicking on any of them will try to switch to that branch and restart the application.  
  This will allow me (or any other contributor) to have all interesting development in a dynamic menu