… being installed.
There is already most of the main code logic for this, though now plug-ins need
to be in their own subdirectories, which breaks for plug-ins/common/ and
plug-ins/python/, while I needed plug-ins in both these categories to generate
the Windows installer welcome images (file-png, and python-fu-eval in
particular).
Once again, meson was not very helpful, since all its functions still refuse to
output generated files in subdirectories, so I end up duplicating plug-in files
with a custom Python script.
This should fix the CI. It was working on my machine as GIMP was installed, but
such a build rule should work even without GIMP installed.
This will also be useful in the future when we'll want to run unit tests of
plug-ins through the finale GIMP binary itself.
