Add support for $desktop, $desktop_number and $desktop_name

Add support for $desktop, $desktop_number and $desktop_name

Should work with any window manager that supports the
freedesktop.org Window Manager Specification
(see http://www.freedesktop.org/wiki/Specifications/wm-spec).

I have tested succesfully with openbox but some more testing with other
wm would be needed!

Support is built-in; if it is not acceptable to loose cpu cycles for this
at every conky refresh step than it should be conditionally compiled.