XfceIconTheme added

[Brian J. Tarricone]

ok, so i finished XfceIconTheme and committed it.  i don't have the 
proper autotools on my laptop, so if someone could run autogen.sh on 
libxfcegui4 and commit the generated files, that would be much appreciated.

i haven't yet ported xfdesktop (or the panel) to use this; that'll wait 
until tomorrow when i'm a bit less tired.

from what i can tell, it should work fairly well, and use the XDG 
basedir spec to find icon directories, with the exception that it will 
make sure that xfce's prefix (i.e., $xfce_datadir/icons/) is in the 
search path no matter what.  it also searches $xfce_datadir/pixmaps and 
/usr/share/pixmaps (if it's a different dir) as a fallback.

you can look at my little test app for the basic usage model:
http://spuriousinterrupt.org/projects/xfce4/files/xit.c
if you're not interested in filenames, and just want a GdkPixbuf, use 
the xfce_icon_theme_load*() set of functions rather than 
xfce_icon_theme_lookup*().

the only thing that needs to be done (aside from debugging) is make it 
support the "Inherits" property from index.theme files.

    -brian