Improve message delegate code, fix multiple issues

- Fix the ListView not auto-scrolling like it should when message
  delegates changed height, such as when images were loaded

- Fix messed up delegate positions overlapping each others when
  movements happen

- Fix previous/nextItem binding broken due to imperative modification

- Fix "messageBodyWidth" binding loop

TODO.md

@@ -49,18 +49,15 @@
 
 ## Issues
 
+- Room pane slightly overlaps chat at small width
 - invisible uploaded mxc images?
 - first undecryptable message
 - Join button 502
 - Leave box button focus
 
-- two upload delegates height bug
 - Pausing uploads doesn't work well, servers end up dropping the connection 
 - Pause upload, switch to other room, then come back, wrong state displayed
 
-- Messed up message delegates position
-- Event delegates changing height don't scroll the list
-
 - In the "Leave me" room, "join > Hi > left" aren't combined
 - When selecting text and scrolling up, selection stops working after a while
   - Ensure all the text that should be copied is copied