[window] Correct get_preferred_height
[window] Correct get_preferred_height

We were setting the height instead of incrementing it when getting the
child height.

clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx