Issue #6112: Add support for loading .icns files

This patch attempts to add support for loading .icns files per the feature request. It ports a plug-in made by Brion Vibber (licensed under GPL v2) to work with the current development version of GIMP.

The original version of the plug-in only supported formats available up to 2004 (roughly to the first release of OSX). Subsequent versions largely store the icons in PNG or JPEG 2000 formats. The code was updated to support these formats by passing the bytes to the PNG and JP2 plug-in via procedures.

There are additional design challenges with exporting .icns files (e.g. the width and height are preset based on what format the icon is saved at and can't be changed), so I wanted to split this fix into two parts to iron out any general issues first (like creating a new plug-in and setting up the build rules properly).

Sample files:

https://gitlab.gnome.org/GNOME/gimp/uploads/cf4b6cab3d8f5f76047eed1e367842c0/GIMP.icns,

https://gitlab.gnome.org/Infrastructure/gimp-macos-build/-/blob/master/package/xcf.icns,

https://github.com/tinalatif/flat.icns/tree/master/icns

(Also, I just thought it would be nice if GIMP could open its own MacOS icon)