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.

st.h

@@ -74,6 +74,7 @@ typedef union {
 	uint ui;
 	float f;
 	const void *v;
+	const char *s;
 } Arg;
 
 void die(const char *, ...);