Just re-arranging on every focus change would cause useless/needless
re-arranges (which have no effect except to waste CPU time). Thus, this
adds a special (undocumented) flag on layouts that makes sure that a
rearrange occurs when the focus changes.
Fixes: https://github.com/awesomeWM/awesome/issues/1799
Signed-off-by: Uli Schlachter <psychon@znc.in>