Use move-rows instead of reinserting then in SessionModel

57485cef includes an example of how it could be used, but it sadly segfaults :D

This would pave the way to get rid of the singleton messagemodel.

After 7ec4321d the moving works (i.e. sessions switch places without crashing), but there are some issues:

• Switching between archived/active sessions crash with !308 (merged) (due to missing .or(Some("")) here)
• Groups don't always get refreshed (perhaps some data_changed() index mismatch?)
  • When a new message arrives to a group, it shows in archived, even though it's not
  • Group visuals and the conversation that opens don't match after that
• Upstream Whisperfork expose-move-row-listmodel