VCS/Redesign file/import structure

app is now cleaned from all VCS stuff,
info too.

the vcs module is very small and contains
vcs.app_is_git_controlled()

vcs is initially imported in main.py,
and when appropriate loads vcs.apprepo,
the class managing Frescobaldi git.

apprepo.py, gitrepo.py and abstractrepo.py
are now independent files that can be imported
when necessary.
When someone wants to implement classes for
other VCSs he will have to add a new file with a
class subclassing AbstractVCSRepo from
abstractrepo.py