[combo-box] Add focusable support
[combo-box] Add focusable support

Allow MxComboBox to accept the keyboard focus.

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