mouse shortcuts: allow same functions as kb shortcuts

Previously mouse shortcuts supported only ttywrite.

This required adding an "Arg" function ttysend - which does what the
original mouse shortcuts did.

config.def.h

@@ -155,9 +155,9 @@ static unsigned int defaultattr = 11;
  * Beware that overloading Button1 will disable the selection.
  */
 static MouseShortcut mshortcuts[] = {
-	/* button               mask            string */
-	{ Button4,              XK_ANY_MOD,     "\031" },
-	{ Button5,              XK_ANY_MOD,     "\005" },
+	/* mask                 button   function        argument */
+	{ XK_ANY_MOD,           Button4, ttysend,        {.s = "\031"} },
+	{ XK_ANY_MOD,           Button5, ttysend,        {.s = "\005"} },
 };
 
 /* Internal keyboard shortcuts. */