Begin work migrating TestWindow demo

[Blquinn]

NOT READY

Begin attempt to port TestWindow demo over to gtk4.

Left a lot of commented out and broken code in this commit.

[dejlek]

Rebase and resolve conflicts? :)

[Blquinn]

@dejlek I've done what you asked, when do I get paid? ;)

Keep in mind I've definitely broken the demo here to get it to compile.

[dejlek]

I wish there was some feedback from @MikeWey here. :)

[MikeWey]

Left a lot of commented out and broken code in this commit.

Is that just the old code still being there, or are large parts of the demo broken?

[EsoCoding]

I was confused for hours. I thought i was installing gtkd-3 instead of 4, because the demo code from TestWindow showed errors of vbox and some others. No problemo, but the "master" fails building right now when running make without arguments. It was later when i took a more close inspection, that it was gtkd-4, but the demo code was old :P