[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.

client.h

@@ -51,5 +51,7 @@ extern void getstate(Client *);
 extern void grav_map_frame_to_win(Client *, int, int, int *, int *);
 extern void grav_map_win_to_frame(Client *, int, int, int *, int *);
 extern void setclientstate(Client *, int);
+extern void reparent_client(struct _Scrn *s, Client *client);
+
 
 #endif