This new job resulted in a package which allows to run GIMP on Windows
(as tested in a VM; at least it starts, I can create a new canvas and
paint). Of course I think this will need to be tweaked a little bit
more, as I'm sure we miss things here and there.

At the very least, even though I add the Python and Luajit binaries,
GIMP on Windows didn't find them. This will need to be investigated.

Also it looks like opening from a remote location may not work. Not sure
if this about a missing GIO module or maybe something which works
differently on Windows (I was not even able to drag'n drop from the
browser!). Anyway this needs to be looked at as well.

Note that gdk-pixbuf-query-loaders is apparently unneeded when GIMP is
built this way (unlike with our crossroad build).

All this to say that this is still an early attempt to full CI build for
Windows.
It doesn't invalidate the crossroad build, because cross-compilation
builds from Linux will always stay very important for Linux developers
to be able to easily fix Windows bugs too; yet the crossroad build has 2
major issues:
1. We haven't figured out yet how to run GObject Introspection tools for
   cross-builds, so the crossroad builds are not full-featured (and this
   is quite a major feature we are missing!).
2. Also I will want to run the installer in the CI at some point and the
   one we use can only run on Windows itself AFAIK. We could try to run
   it through Wine, but still anyway the point 1. is already quite a
   blocker, let's do the simple thing.

Note that we will likely want to move to meson for this build, because
autotools is very slow on Windows. But as long as the few blocker meson
bugs are not fixed, let's stick to the slow yet good build.