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
As per https://github.com/matrix-org/matrix-doc/issues/1354
This is whitelisted to only jitsi widgets for now as per comment,
mostly because any widget that we may make always-on-screen we need
to preemptively put in a PersistedElement container, which is
unnecessary for any other widget.
Apologies that this does a bunch of refactoring which could have
been split out separately: I only discovered what needed to be
refactored in the process of doing this.
Fixes https://github.com/vector-im/riot-web/issues/6984
