Create a content provider instance to get message content models from

This means that all content models will now come from the same source to remove duplication across multiple models and `chatbarcaches`.

It also handily breaks the dependency on needing `MessageContentModel` for `NeochatRoom`

src/timeline/MessageDelegate.qml

@@ -190,6 +190,7 @@ TimelineDelegate {
 
     Message.room: root.room
     Message.timeline: root.ListView.view
+    Message.contentModel: root.contentModel
     Message.index: root.index
     Message.maxContentWidth: contentMaxWidth