Ensure deselection occurs after deleting an element.
Fix undo after creating a node or link
Node tool should not select existing elements.
Force vertical scrollbar to be displayed, to prevent the feeling of the view moving around (which is caused by the selection view appearing / disappearing).
Fix undo for delete node / link and for move node.
Updated todo list.
Fixed undo for change property.
Prevent selection from creating redundant change property events.
Redraw a node's label when its properties are changed.
Don't select a node's label; only select its grouped path(s).
Factored out NodeCanvasElement and LinkCanvasElement
Reorganised todo list.