Drive-by: convert to enum singleton.
Comment tweak.
Added layer as an argument to addEntry().

Previously entries were added, then had their layer set. But to
support alternate base layers, we need to know the layer at the time
of adding, so that a conflicting tile can either be removed or
stashed in its layer (this is not implemented yet).
Added TudeySceneModel.addObserverLast()

I need the layer tool to be the last observer so that
it can hear about a new entry after the entry's layer
has been set. I started to do some new OrderedObserver
interface, but reordering the existing observers is a pain
and so while this is a little funny it's much much much less of a hack.
Comment.