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.



There is a ton of code duplication between this and the normal batterymon executable. Could you refactor this so the core functionality is separate from the GUI? If you don't want to do that, I can refactor the existing branch, then you can build the gtk3 stuff on top of it.