When setting, delete the old one. Because delete checks if it exists first, we can safely call this.
The change in FromWidgetPostMessageApi is just something noticed while debugging.
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
