The File menu now includes a "Create" submenu, and Delete/Rename
options. Nodes can offer Delete/Rename functionality, and the
options on the menu are automatically enabled/disabled based on
the selected node. Each node can also offer Create functionality,
and specify a list of node types (including itself) from which the
option should be made available. The menu is dynamically generated
based on the selected node.
The Context menu on the treeview works in a similar way, except that
nodes can offer any context menu items (we don't allow this on the
top menu, as that should stay consistent to avoid user confusion).
