Make mouse event structure clearer by defining events (up, click, drag)

and simplifying how buttons and wheels are represented, from Ailin
Nemui. Should be no functional changes.

window.c

@@ -1011,7 +1011,7 @@ window_pane_mouse(
 		    options_get_number(&wp->window->options, "mode-mouse"))
 			wp->mode->mouse(wp, sess, m);
 	} else if (wp->fd != -1)
-		input_mouse(wp, m);
+		input_mouse(wp, sess, m);
 }
 
 int