Make Frescobaldi restartable

1. Make Frescobaldi restartable  
  Make life easier when working on Frescobaldi code
2. Test if Frescobaldi is run from a Git repository  
  This will enable presenting the corresponding GUIs only for Git users
3. A first example for 2. ;-)

In itself this pull request doesn't provide too much useful functionality.
But I would love to see a GUI that allows me to easily switch between different versions of Frescobaldi (i.e. different branches of my local Git repo or pull requests on other remotes