[Xmonad] Popup status bar support

[Donald Bruce Stewart]

dons:
> Proper support for a popup statusbar has been added  -- a whole 1 line
> needed rewriting :-)

Another, and hopefully final, patch has landed for this, generalising
the top-of-screen gap for statusbar support to arbitrary sides of the
screen, and for arbitrary physical screens. You can thus have several
different statusbars/docks on any edge of the screen, and on any
physical screen.

-- Don