xfdesktop menu cache

[Brian J. Tarricone]

hi all-

i just committed a caching system for the xfdesktop menu.  basically, as 
it's creating the menu, the cache creates a tree structure (using 
GNodes) to describe the menu.  after the menu's done, the tree gets 
written to an XML file, in the same format as ~/.xfce4/menu.xml.  in 
another file, i store a list of all the relevant menu files and .desktop 
directories along with their mtimes, to check for cache validity.  on 
startup, xfdesktop reads this file, checks the mtimes, and matches them 
against the values in the file.  if there are filesystem changes, we 
redo the menu, if not, we simply parse the cached XML file instead of 
~/.xfce4/menu.xml.

just to confuse the issue (^_~), the cache files are stored in 
~/.cache/xfdesktop.  i'll change it when we decide on what to do about 
the config dir structure.

this could use a lot of testing, though it appears to work ok for me.  
happy bug-hunting!

on a side note, i've been urged to do the menu generation in a separate 
thread.  i don't think this will buy us anything, because the menu 
generation is very gtk-heavy, so i'd have to wrap the _entire_ thing in 
a big gdk_threads_enter()/gdk_threads_leave() pair, and any other 
gtk-related stuff in the main thread would get blocked anyway.  as it 
stands, the initial menu generation (when xfdesktop starts) is idled via 
g_idle_add(), and xfdesktop connects to the session manager before this 
runs, so it's about as fast as it can be, IMO.  if anyone has any better 
ideas (aside from rewriting it to be faster - hah), please let me hear 
them.

	-brian