battstat: Add GTK3 support

Code taken from:
https://git.gnome.org/browse/gnome-applets/commit/battstat?id=274f17fe92b98b5a0bdd31de734240b4cc6227e9
https://git.gnome.org/browse/gnome-applets/commit/battstat?id=137c00e2de6098b0ecef75ec208d58de138d7a82
https://git.gnome.org/browse/gnome-applets/commit/battstat?id=13522dcb2cb272b11214728c69c9ec179de78e2d