fix: improve webxdc data cleanup and fix closing and cleaning up webxdc apps when chat is deleted.

[Simon-Laux]

changes:

• save last bounds also on startup not only when closing app.
• cleanup old webxdc app webdata of webxdc apps that don't exist anymore
• cleanup webxdc data when deleting a chat (workaround for missing webxdc instance deleted event in that case, WebxdcInstanceDeleted event is not emitted while deleting a chat chatmail/core#6670)

closes #5758

I tested that this works correctly when deleting single messages, that it works with ephemeral messages and when deleting a chat.

this pr needs a core with chatmail/core#7579 merged.

[Simon-Laux]

we could also think about whether we also want to run the cleanup logic once on startup.