plug-ins: add HEIF loading/saving plug-in written by Dirk Farin
Thanks a lot to Dirk for contributing this, added him to AUTHORS. Import the code from https://github.com/strukturag/heif-gimp-plugin.git as of today. Merged the files into a single-file plug-in. Changed the code a lot to match our coding style, but only formatting, no logic changes. Still uses deprecated GimpDrawable API and no GIO, but I wanted to do actual code changes separately from the initial import. Also disabled metadata support because updating that to GimpMetadata was too much for the initial import.
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