Port to Gtk 3.12.

Signed-off-by: Maël Lavault <mael.lavault@mailz.org>
Update bleachbit.desktop
Resolve conflict
Merge branch 'moimael-modernization' into modernization
Fix syntax error
Port to GtkApplication, GtkApplicationWindow AppMenu and HeaderBar.

Signed-off-by: Maël Lavault <mael.lavault@mailz.org>