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()

harmonyqml/components/Chat/Chat.qml

@@ -8,7 +8,7 @@ HColumnLayout {
     property string roomId: ""
 
     readonly property var roomInfo:
-        Backend.models.rooms.get(userId).getWhere("roomId", roomId)
+        Backend.models.rooms.get(userId).get(roomId)
 
     property bool canLoadPastEvents: true
 
@@ -17,7 +17,7 @@ HColumnLayout {
 
     RoomHeader {
         displayName: roomInfo.displayName
-        topic: roomInfo.topic
+        topic: roomInfo.topic || ""
     }
 
     RoomEventList {}