checkbutton: Fix redraw issues

checkbutton: Fix redraw issues

This is a hack to get around the optimizations done by the CSS engine.

The CSS engine will notice that no CSS properties changed on the
widget itself when going from one state to another and not queue
a redraw.
And the reason for no properties changing will be that only the
checkmark itself changes, but that is hidden behind a
gtk_style_context_save()/_restore() pair, so it won't be caught.