Rework the alt-tabbing code to be a lot simpler.

Diff mostly from Edd Barrett, with some minor changes from me.
Unfortunately the issue where apps like gvim and xpdf are stealing
keyrelease events causing the ordering to be messed up, but this is a
lot better. A fix for the aforementioned issue shall be forthcoming,
once a good one's been found.

ok okan@, also tested by todd@

screen.c

@@ -69,7 +69,4 @@ screen_updatestackingorder(void)
 void
 screen_init(void)
 {
-	struct screen_ctx *sc = screen_current();
-
-	sc->cycle_client = NULL;
 }