There were three edge cases:
1) The wrong message was replaced:
   - You send a message
   - You open the edit menu
   - Contact or another device of you sends a message
     (lastMessageId is updated)
   - You finish your edit and a message is sent to replace your contacts
     message, which is obviously not what we want. Database and XMPP
     both get the wrong message id to replace.
2) Can't edit last message:
   - You send a message
   - Contact sends a message
     (lastMessageId is updated)
   - You can't edit your last message, although that is expected to work.

The solution was to only update the lastMessageId for own messages.

Also the QML code has been slightly beautified by the use of a signal.