Version the GdkX11 import, fixes #4965

The unversioned import results in the following log noise:

    /usr/lib/python2.7/site-packages/sugar3/graphics/window.py:26: PyGIWarning: GdkX11 was imported without specifying a version first. Use gi.require_version('GdkX11', '3.0') before import to ensure that the right version gets loaded.
      from gi.repository import GdkX11

src/sugar3/graphics/window.py

@@ -20,6 +20,9 @@
 STABLE.
 """
 
+import gi
+gi.require_version('GdkX11', '3.0')
+
 from gi.repository import GObject
 from gi.repository import GLib
 from gi.repository import Gdk