quick frontend (app): new synchronisation mechanism: on some frontend (notably on mobiles) frontend can be paused and lose synchronisation with backend, this can also happen if backend restart a connexion without resuming using stream management. To manage that, QuickApp now has a sync attribute which is True when the backend is in sync with frontend. When synchronisation has been lost and is possible again, all widget will have their "resync" method called, which can be used to do necessary calls to update the status. It's the responsability of each widget to then reset the "sync" attribute, once everything has been resynchronised.