port from pygtk to pygobject

To allow the tray icon to be used on python3.

see: [Porting_PyGTK_to_PyGObject](https://wiki.gnome.org/action/show/Projects/PyGObject?action=show&redirect=PyGObject#Porting_PyGTK_to_PyGObject)

On the other hand, that implementation is GNOME3 which is extremely ugly and deprecates icons, as far as I can see.

However, more dependencies may be eliminated and unified by porting to PyGObject:
- PyGTK => [gi.repository.Gtk](http://lazka.github.io/pgi-docs/api/Gtk_3.0/index.html) + [gi.repository.Gio](http://lazka.github.io/pgi-docs/api/Gio_2.0/index.html)
- pynotify => [gi.repository.Notify](http://lazka.github.io/pgi-docs/api/Notify_0.7/index.html)
- dbus-python => [gi.repository.UDisks](http://lazka.github.io/pgi-docs/api/UDisks_2.0/index.html)
- zenity, see #32 

Leaving only 3 dependencies: PyYAML, UDisks, PyGObject.
Which means, this is probably worth.