xfdesktop menu cache fixes

[Brian J. Tarricone]

hi all-

i just made some changes to the menu caching system so it works properly 
in a few important cases that i didn't test properly.  anyway, the cache 
config file name changed, so it shouldn't be a problem, but just to make 
sure, please do 'rm ~/.cache/xfdesktop/*' after updating.  also, benny 
made me a patch that changes how xfdesktop looks for .desktop files, so 
if any of your apps are missing, that's probably why - you'll need to 
set some env vars to make it work properly.  i'm not sure exactly how 
that works, perhaps benny can explain that.

    -brian