quickSettings: Update bind constraint offset on idle

Use Meta.LaterType.BEFORE_REDRAW to queue setting the bind constraint offset in an idle callback. This is needed since the signals that trigger updating the offset may be emitted during allocation, at which point queuing new relayouts isn't ideal, since it could result in relayout cycles. In this case, we really do want to relayout as a side effect of another actors allocation, so make this explicit.

This fixes a few warnings such as:

The actor '[:0x3138d70]' is currently inside an allocation cycle; calling clutter_actor_queue_relayout() is not recommended