Import GDK in preparation for GTK3 conversion

Some parts of GTK moved to GDK, so we need to import the latter for things to
work after the conversion script runs.

Signed-off-by: Sascha Silbe <silbe@activitycentral.com>

src/sugar3/graphics/palettewindow.py

@@ -23,6 +23,7 @@ STABLE.
 
 import logging
 
+from gtk import gdk
 import gtk
 import gobject
 

src/sugar3/graphics/style.py

@@ -25,6 +25,7 @@ STABLE.
 import os
 import logging
 
+from gtk import gdk
 import gtk
 import pango
 import gconf

src/sugar3/graphics/window.py

@@ -21,6 +21,7 @@ STABLE.
 """
 
 import gobject
+from gtk import gdk
 import gtk
 import warnings