[amiwm] Implement a keyboard shortcut to move a client to a different screen

This allows a keyboard shortcut to move a client to a different amiwm
screen.  That way when you have things like Firefox restart every window
in a single screen, you can quickly move screens to where they should be.

module.h

@@ -17,6 +17,7 @@
 #define MCMD_ROTATE_WINDOW_RAISE 19
 #define MCMD_ROTATE_WINDOW_LOWER 20
 #define MCMD_UPDATE_BATTERY 21
+#define MCMD_WINDOW_MOVE_NEXT_DESKTOP 22
 
 struct mcmd_header {
   XID id;