This commit does a number of things which I could not subdivide into
smaller commits.
* Converts ETreeModel to an interface, implemented by MessageList.
* Drops ETreeMemory and ETreeMemoryCallbacks, which were ETreeModel
subclasses. Their functionality is subsumed by MessageList.
* MessageList drops its public ETreeModel pointer, since MessageList
now implements ETreeModel as an interface.
* Adds message_list_set_expanded_default(), which takes over for
e_tree_memory_set_expanded_default().
