widget: ensure the "disabled" pseudo class is set when needed
widget: ensure the "disabled" pseudo class is set when needed

Make sure the "disabled" style pseudo class is set when the widget is
disabled.

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