UNDO
'Backspace' key to delete (what?).
http://www.daa.com.au/pipermail/pygtk/2003-October/006182.html
support shift-left for closing, Left for NOT closing.
support coloring IDs if focused
support 'Delete' key. [shift-delete works] AT LEAST FOR THE SIMPLE NO-CHILD CASE
auto-convert if there is a paren in a newly edited expression?
support 'Insert'-after key (should be the default)
	Ctrl-Insert is taken but it's kinda obscure.
	Alt-Insert is free
	just Insert is now insert-child, that shouldn't be so ... common?
left-left-left -> error
treeview/model if node edited and empty, remove.