[Xmonad] Popup status bar support

[Donald Bruce Stewart]

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

Assuming you have something like dzen[1] running, say as:

    while true ; do date ; sleep 30 ; done \
        | dzen2 -ta r -fg '#a8a3f7' -bg '#3f3c6d' -p &

Which displays a 15 pixel high status bar at the top of the screen,
you'd edit your Config.hs, change defaultStatusGap from 0 to 15, 
and reload. mod-b will then toggle on or off the status bar.

Screenshots:

    http://www.cse.unsw.edu.au/~dons/code/xmonad-web/images/screen-rob-status-spiral.png
    http://www.cse.unsw.edu.au/~dons/code/xmonad-web/images/screen-dons-status.png

The 'gap' is just a pixel gap, so you could put any permanent (or pop
up) content in that gap. Any client that ignores the window manager
would be suitable to use as a 'status bar'.

By the way, the 'gap' height for a given font is calculable using the
script in utils/gapcalc.c, courtesy of Rob, dzen's creator.

-- Don

1. http://gotmor.googlepages.com/dzen