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.
ba149f1759