[client] x11/sdl/wayland: implement new minimize (stub for wayland)

This implements the new minimize function introduced in the last commit
for x11 and SDL. Wayland at current is just a stub and needs some
attention.

client/displayservers/Wayland/shell_xdg.c

@@ -146,3 +146,8 @@ bool waylandGetFullscreen(void)
 {
   return wlWm.fullscreen;
 }
+
+void waylandMinimize(void)
+{
+  //FIXME
+}