Merge: GTK clean up and a few new features

You should read this PR commit by commit as the style update touches almost every line. The style update adds and removes spaces, and it removes empty and commented methods. It does not touch the documentation, there is still a lot of work to do there.

Please read the commit description of "lib/gtk: ignore deprecated warnings" for the rationale of the change.

GTK will be used to implement the abstract UI on GNU/Linux.

Pull-Request: #1312
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>