Gtk3 support

Hello,

I added Gtk3 support, through gobject introspection in python3.

The main changes are in *batterymon-gtk3*.

Please note that right click on the tray icon is currently not working.



Perhaps refactoring is better, so that the existing executable keeps working as expected... I will be busy until the next weekend with the courses at college... let me try to refactor after that (y)