ui: ensure that Gtk 3.0 is the version loaded

lib/solaar/ui/__init__.py

@@ -22,10 +22,14 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 from logging import DEBUG as _DEBUG
 from logging import getLogger
 
+import gi
+
 from gi.repository import GLib, Gtk
 from logitech_receiver.status import ALERT
 from solaar.i18n import _
 
+gi.require_version('Gtk', '3.0')
+
 _log = getLogger(__name__)
 del getLogger