CODE: rewritten listvalidate to make easier to use fallback converters

Ora now fallback to calligra. If no satisfactory converter found, artscript
will not load the image into list.
FIX: typo on png compression filter
FIX: Removed "calligraconvert always trows error" msg

Added mimetype option to avoid ambiguity
FIX #60 : XCF gamma is wrong

Script-fu ignored colorspace information. We now transform the render input
to rgb, this correctly makes gimp script aware of colorspace ops.