prelimiary xfdesktop multi-screen support, crashing bug fixed

[Brian J. Tarricone]

hi all-

i reworked a bit of xfdesktop so i can move toward supporting people 
with multiple screens.  by this i mean multiple logical screens on a 
single X display, via :0.0, :0.1, etc.  my plan for people that run 
multiple displays is that you'll need to run multiple instances of 
xfdesktop.  i'm not sure how this relates to xinerama as i have no idea 
how that works (yet).

as an intended side-effect, the app-crashing bug appears to have 
disappeared.  my guess was that it had something to do with the global 
|XfceDesktop xfdesktop| variable, and it appears i was right.

i _really_ need feedback from people with multiple screens.  currently, 
things aren't very useful.  settings from the desktops on different 
screens will overwrite each other in not-terribly-useful ways.  i know 
how to fix this, it's just a matter of actually doing it.  also, i'm 
betting that the windowlist and app menus won't work properly, because 
they keep around a local copy of only a single NetkScreen (right now, 
it's the NetkScreen of the last desktop initialised).  this is fixable 
as well, but it's just something i need to do.

basically, xfdesktop now creates and paints a fullscreen window for 
every screen present, and it'll put the same backdrop image or color on 
all of them.  if people have any links to any documentation or 
information that might be useful about multi-screen stuff, i'd 
appreciate some pointers.  as you might guess, i don't have a 
multi-screen setup, so i have no idea if the stuff i do actually works.

so, that's my major accomplishment for the night.  well, that and 
cleaning up my resume and writing a few cover letters.  whoopie do, how fun.

    -brian