[Pmt-exif] splitting PNG/Exif proposal

[Glenn Randers-P. <gl...@ho...>]

The work we are doing on defining the syntax for the textExif chunk
may be useful outside the specific task of recording Exif information.
There are several other applications that have been mentioned in the past
on png-list or directly to me, e.g., chemical structure information and
geographical coordinate system data.

Therefore it might be a good idea to split the PNG/Exif proposal into
two documents, one relating specifically to Exif and its subkeyword list,
and another dealing with the subkeyword=value syntax.  The latter could
eventually become a part of the PNG Extensions document.

(Draft 0.13 talks about "keywords" instead of "subkeywords" but I'm
going to change that, so that "keyword" only applies to the text chunk
keyword ("Exif", in the case of the textExif chunk) and all the rest
are "subkeywords").

Glenn