#165 Add GtkTreeView sidebar for easy access to notes

[Mihail Atanassov]

Xournal's awesome, except for that little thing: getting to your notes is a bit of a pain. If a tree view were added as an optionally enabled sidebar, then that would solve 2 problems:
1. It will allow for easier management of your notes,
2. It will avoid creating a hierarchy on top of the file system.

Having multiple tabs, or better yet, auto-save on close for existing files (config-able), would most definitely complement this addition.

I'd be happy to assist with testing (it shouldn't be too big a change). If nobody is willing to add that, I can also try my best to submit a patch summer 2013.