GIMP XCF extension, PSD and other layered formats

PSD export would be useful in inkscape. Currently the only available
multi-layer format is XCF. PSD is much more widely supported than XCF and
inkscape doesn't need any specific features of XCF (a format which even the
gimp developers discourage other projects from using).

I had vaguely intended to do this myself but since gimp_xcf.py doesn't work
on windows and I dont have a decent development setup I'm filing this
request instead for now. (And thanks to Aaron Spike for the
encouragement)

Probably only the bare minimum of changes are required to get PSD support.
All that is likely needed is to change any occurances of "xcf" to "psd" in
the existing script and I'm 99% certain that will work (although that
touch/rename a few variables too). Aaron has said he would much rather
avoid two copies of the script, but in the short run that might be a good
idea so as to get the feature included and out to users.
If the script were reorganised it could potentially make it easier to offer
other formats like layered TIFF or maybe even MNG and other formats through
the same process, and Aaron thinks he might take that approach, if and when
he has the time.

There are a few other cosmetic details like renaming MyEffect to something
more descriptive like GIMP_Export_Something. In gimp scripts I would
always avoid abbreviations and instead of "img" I would write "image" (who
knows who will be reading the code, makes sense to keep it as simple as
possible).

If I ever get a decent developement box setup I'll take a look at improving
back compatibility or at least creating a seperate version that can work
with gimp 1.x and Cinepaint. If the script-fu part of the script were
further modularised it would make it easier, the gimp scripts have only
changed slightly over the years. Future proofing and testing with Tiny-Fu
will probably be needed too.
(A quick and dirty native C or C++ implentation of PSD support might be a
whole lot easier to be honest but I happen to know enough python and scheme
to do things this way if I could just get things organised.)