stylecontext: If we force invalidate, invalidate all properties

stylecontext: If we force invalidate, invalidate all properties

We only FORCE_INVALIDATE when something weird changes that the CSS
machinery can't detect. But now that our style_updated functions skip
recomputations when some properties don't change we want to make sure
these recomputations are still run. So we just claim all properties
changed.

http://bugzilla.gnome.org/show_bug.cgi?id=695482