Rewrite most of grab_menu in grab.c (it was partly 9wm code).

This should work functionally the same, with a few simplifications.

Changes:
- we don't care if you're holding another button when you release the
  menu key if you don't want to select anything, move off the menu.
- remove the hysteresis from the menu selection (before you had to move
  more than three pixels onto a new menu entry before it selected it)
- simplify a lot of the selection code
- kill dead code.
- do what the XXX comment said and cache the screensize (i may tweak
  this later).

As far as I can tell, the only code remaining from 9wm is the list of
fonts in calmwm.c. Others appear to concur.

ok marc@, looked over and tested by a few others. Reminders from okan@.

group.c

@@ -92,7 +92,7 @@ _group_purge(struct group_ctx *gc)
 	struct client_ctx *cc;
 
 	if (gc == NULL)
-		errx(1, "_group_commit: ctx is null");
+		errx(1, "_group_purge: ctx is null");
 
 	TAILQ_FOREACH(cc, &gc->clients, group_entry)
 		if (cc->groupcommit == 0)