Make app.is_git_controlled() cheaper
Add git module and test file

git module contains:

Git(object) class implementing functionality to manage
a git repository.
Currently this is only the repo Frescobaldi lives in, but later
we will also use this for managing project repos.

GitApp(Git) class used for the
app.repo
object representing Frescobaldi's repo.
Implements app.repo.upstream_remote()
Show Git branch/remote in Window title bar