ETreeViewFrame embeds a GtkTreeView in a scrolled window and adds an
inline-style toolbar beneath the scrolled window which can be hidden.
The inline-style toolbar supports "add" and "remove" actions, as well
as move actions if the tree view is reorderable and selection actions
if the tree view supports multiple selections. The action set can be
extended through e_tree_view_frame_insert_toolbar_action().
This also adds a small demo program: test-tree-view-frame
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().
