Enabled deletion of root nodes (except last). Added disabled state to context menu and toolbar.

Toolbar binds to tree selected_node and rechecks the toolbar button
action for the selected node.
Fixed bug in create_node for nodes requiring a new sub menu

(i.e. when creating the first child for a node) caused javascript
update to fail silently and hence not update until the next ajax poll
returned.
Refactoring xmldb into separate "document" and "xmldb" classes.

Needs testing!