Make app.is_git_controlled() cheaper

when run first, add a flag to the qApp object,
later simply return this
Move git-branch titlebar setting to mainwindow.py

app.caption is called often and from many widgets.
So setting the git-branch in the title bar has to be in the mainwindow itself.
Add (empty) GitError Exception class
Clean up git module

The functions in git.app now return more 'atomic' results.
If one wants to combine elements (e.g. in the App titlebar)
one should call the atomic functions and process them outside.
Add Git.remotes()
Add GitApp(Git) subclass for Frescobaldi

Subclass to manage Frescobaldi git vs. projects' git.
Implement upstream_remote() to point to Wilbert's official repo.

TODO: Fix Git._read_config(). This is totally wrecked obviously