Implement always-on-screen capability for widgets
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
