Add support for specifying an abbreviated description for enum/
flags values, which can be used in contexts where the full
description is too long.

Since the exact layout and size of Gimp{Enum,Flags}Desc is part of
the ABI, we can't simply add a field to these structs to hold the
abbreviated description.  Instead, we use the fact that entries
with a repeated value in the value descriptions array are ignored,
and that the array is NULL terminated (in particular, that all non-
NULL entries are followed by at least one additional entry), and
specify the abbreviation in the "value_desc" field of the entry
that immediately follows the initial entry for a given value,
setting the "value" field of both entries to the same value.

Right now this behavior is undocumented, so there is no proper way
to specify abbreviated descriptions in the API, and is only meant
to be used in generated enum files.