Issue #6112: Add export support for ICNS

This patch attempts to resolve the other half of #6112 (closed) and add support for exporting ICNS files. Currently it chooses the oldest "modern" ICNS format for a particular layer size and uses this for export (via the file-png-save procedure since modern ICNS formats are all either PNG or JPEG 2000). Since all "modern" types store PNG data with no differences beyond the type label, this should help ensure icons exported from GIMP are useable by default.

As ICNS uses predefined icons based on the type label, the export GUI indicates which layers will not be exported without adjusting the layer size:

Feedback on the GUI is appreciated! Another suggestion was to highlight any invalid icons with a distinct border. Is there an existing CSS style/class color that should be used for that purpose?

It is hoped that this patch could also facilitate resolving #5507, as it would be possible export the GIMP icon from source to .icns.