wmaker: rewrote the update of _NET_WORKAREA property
As reported by Moritz, the Qt toolkit (and maybe a few other ones) are
using the content of this property, but WindowMaker did not set it properly
so for examples the menus in KDE could fail to display in multi-screen
configuration (probably because the toolkit is using the property to make
sure the menu stays in the visible area).
The original code just assumed it was safe to use the usable area of the
1st screen in the list, but unfortunately it is not that simple.
The new code calculates a rectangle that contains the usable region from
all the screens, so the toolkit gets a more accurate value.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>