Improved ListModel and ListItem

- New ListItem implemented using a metaclass, which makes defining new
  ListItem subclasses much cleaner and shorter

- ListModel functions taking an index now accept either an int
  (list index) or str (value of a main key)

- getWhere() gone, simply use get(a_main_key_value) now

- updateOrAppendWhere replaced by update()/upsert()

TODO.md

@@ -5,7 +5,7 @@
 - Bug fixes
   - 100% CPU usage when hitting top edge to trigger messages loading
   - Sending `![A picture](https://picsum.photos/256/256)` → not clickable?
-  - Icons aren't reloaded
+  - Icons and images aren't reloaded
   - HStyle singleton isn't reloaded
   - `MessageDelegate.qml:63: TypeError: 'reloadPreviousItem' not a function`
   - Bug when resizing window being tiled (i3), can't figure it out