Import upstream Diehard4 by obscurant

Verbatim contents of the upstream GitHub archive (Diehard4-master.zip,
commit 254ee204deebc64793123a9643c85b7d113c6c22, files dated 2019-05-21).

Imported unmodified so that every change in this fork is visible as a diff
against the original, which is what GPL-3.0 section 5(a) is asking for.
All code in this commit is obscurant's work, not mine.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

index.theme

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Type=X-GNOME-Metatheme
+Name=Diehard4
+Comment=It's flat and it's blue
+Encoding=UTF-8
+
+[X-GNOME-Metatheme]
+GtkTheme=Diehard4
+MetacityTheme=Diehard4
+ButtonLayout=:minimize,maximize,close