save and restore list position for threaded lists

Followed @grote's suggestion and implemented the list save and restore via the MessageTracker interface. I felt it was best, for now at least, only to implement this feature in the threaded lists, i.e. Forum and Private Gropus, as the user requested. I feel there are additional considerations that we must consider before we implement this in private chats.

Closes #894 (closed) Closes #946 (closed)