Add app.repo.active_branch()

current_branch() may change during Frescobaldi's lifetime
while the application is run from the branch that was
current on startup.
Fix Git.checkout()

add the -q option because checkout returns its result
message to stderr
Add Git.tracked_remote_label()

return either 'local', the name of the remote
or remote/branch if the tracked branch has a different name.
Add Git menu

For now simply list all local branches.
Clicking on a branch tries to checkout this and restart the application.
If git gives an error (e.g. non-clean working tree) it aborts with a message box.
If checkout succeeds the user gets a message box and can restart
Frescobaldi now or later.