This kind-of-reverts 058dbab828.
If banning_refresh() is called, only the lua events that it generated before are
now generated (the unfocus event). The actual mapping and unmapping of X11
windows is defered until the end of the main loop via a new per-screen
need_lazy_banning flag.
Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>