Store node palette in the network document rather than in the GUI state.

Change the representation of the palette of node shapes very slightly.
The palette is now stored as a part of the document (network) itself,
rather than in the state of the GUI. This means when you load a new
document, you automatically get the palette it was built with. Also, a
node can now store either the name of its shape (for lookup in the
palette), or its full description, with the shape name being preferred
in most circumstances.