remove unnecessary IndexableEnum

This is not really critical since list(Type)[i] works just as well. This fixes
a couple other problems from the enum transition:
- The name of TYPES was not really consistent with its new role as an enum
  type. Renamed this ImageType.
- Fix a rather egregious bug in embedart (undefined reference to Image).
fold OrderedEnum into autotag module

This puts the OrderedEnum generic class next to where it is actually used. It
also refers to the recipe it is taken from on docs.python.org. I also took the
opportunity to give this a capitalized name (since it's a proper type).