Make sure an indexed image always has a colormap. This was the case
before, except one could set a NULL colormap via the PDB.

Add gimp_image_unset_colormap(), and make gimp_image_set_colormap()
never set the colormap to NULL, even if NULL is passed. Change the
only places where actual unsetting makes sense to use unset().

Make some GUI places deal gracefully with palettes/colormaps with zero
entries.