Bugfixes, calling stability improvements

* Fix link preview send hangs
* Fix saving decrypted files
* Send nice filenames more often
* Show commands on MUC when present
* Fix showing MUC whispered images as whispers
* Show thread selector when starting a mention
* Support urn:ietf:rfc:3264 (should improve call connection
  stability with JMP)
* Fix destroy-before-initialize lifecycle crash
* Merge calling improvements from upstream
* Many small and crashing bug fixes