acme: mouse movement for Del

If the mouse was in the tag of the old window,
it was most likely pointing at Del. If bringing up a
new window from below and not moving the mouse
somewhere else, adjust it so that it ends up pointing
at Del in the replacement window's tag too.
This makes it easy to Del a sequence of windows in
a column, from top to bottom.

http://www.youtube.com/watch?v=ET8w6RT6u5M

R=r
http://codereview.appspot.com/6558047

src/cmd/acme/dat.h

@@ -239,6 +239,7 @@ struct Window
 	uchar	filemenu;
 	uchar	dirty;
 	uchar	autoindent;
+	uchar	showdel;
 	int		id;
 	Range	addr;
 	Range	limit;