Support UTF-8 key bindings by expanding the key type from int to

uint64_t and converting UTF-8 to Unicode on input and the reverse on
output. (This allows key bindings, there are still omissions - the
largest being that the various prompts do not accept UTF-8.)

window.c

@@ -1102,7 +1102,7 @@ window_pane_reset_mode(struct window_pane *wp)
 
 void
 window_pane_key(struct window_pane *wp, struct client *c, struct session *s,
-    int key, struct mouse_event *m)
+    key_code key, struct mouse_event *m)
 {
 	struct window_pane	*wp2;