Things, to do, in order of priority:

- Editing behaviour that doesn't suck
    Gtk2's default behaviour for editing cells in a TreeView is jaw-droppingly bad
    Need to add sensible behaviour on receiving the following keys:
        - TAB
        - ENTER
        - CURSOR
    This is proving to be more difficult than I'd hoped
    
- Add support for a custom CellRendererSpinButton for numeric values
    How do I do this? Maybe we should let people pass their own renderers?

- Figure out what's up with blocking the size_allocate signal