Add a GTK4 keyboard hint test tool
GTK3 and GTK4 behave differently (see #298 (closed)) so lets have a simple tool to test that as well.
Also improves the GTK3 version a bit.
GTK3 and GTK4 behave differently (see #298 (closed)) so lets have a simple tool to test that as well.
Also improves the GTK3 version a bit.