clutter: Reduce usage of default context

Kind of redoing what @jadahl started in !2017 (closed) & the following up PRs, with a slightly bit more cleanups

There are a lot of commits as i tried to keep it easily reviewable and as it doesn't remove any APIs except ClutterSettings.get_default() which seems to be widely used by extensions. We can keep the API and get the settings from the default context with a deprecation warning. This is now implemented