[combo-box] Set the correct index on key-press events
[combo-box] Set the correct index on key-press events

The index should be decreased when the CLUTTER_Up key is pressed.

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