Make frescobaldi restartable

Solution from here:
http://stackoverflow.com/questions/15221361/how-to-restart-pyqt4-application

Seems to work, but I didn't test it with command line arguments
Add app.qApp.isGitControlled flag

This flag is True if Frescobaldi is run from the Git repository.
The used check is the presence of the .gitignore file
Only create the restart menu item when run from Git
Merge pull request #219 from uliska/restart-app

Make Frescobaldi restartable