xfdesktop new stuff - needs testing

[Brian J. Tarricone]

hi all-
i committed a huge amount of stuff to xfdesktop this morning, and it 
should hopefully address a bunch of multiscreen (non-xinerama) issues.  
i know there will be problems with it, as i obviously can't test it, and 
there are a few things i'm just not sure about.

there's a new settings dialog that needs testing as well.  note: the 
color gradients don't do anything yet.  note2: all your old desktop 
settings are now lost, as the MCS setting names have all changed (mainly 
just to "setting_screennum").  so write them down before updating if you 
care.

so yes, test it.  danny, that means you ^_~.  the crash on exit should 
be resolved, and it should theoretically be able to draw different 
backdrop images on different screens.  theoretically.

unfortunately, the gnome app crash/hang problem still does not seem to 
have been resolved, despite reworking a bunch of stuff i thought might 
be the culprit.  *sigh*

    -brian