Trivial GTK3 porting fixes

Fix some trivial issues missed earlier: various missing imports,
some minor API changes to adapt to, do_size_request simple porting,
etc.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/activity/activity.py

@@ -58,6 +58,7 @@ from functools import partial
 from gi.repository import GConf
 from gi.repository import Gtk
 from gi.repository import Gdk
+from gi.repository import GdkPixbuf
 from gi.repository import GObject
 import dbus
 import dbus.service