Make ActiveWidgetStore clear persistent widgets
ActiveWidgetStore is now reponsible for removing the current
persistent widget from the store if it's been removed from whatever
room it was in. As per comment, this leaves us with the store updating
itself in this case but in all other cases, views call setters on the
store to update its state. We should make it so the store keeps itself
up to date and views aren't responsible for keeping the store up to date.
The store now emits events so it can notify PersistentApp when it changes.
Fixes https://github.com/vector-im/riot-web/issues/7076
