generic GtkIconTheme function, other changes

[Brian J. Tarricone]

hi all-
those of you paying attention to xfce4-commits may notice that i added a 
single-function interface to GtkIconTheme to libxfcegui4:
GdkPixbuf *xfce_load_themed_icon(const gchar *name, gint size);
i already converted xfdesktop to use it, and it seems fine.  i'll do the 
panel later.  jean-francois, if you'd like to have the menueditor use 
it, go for it.  also, i noticed that there was some xml parsing you were 
doing in which you call xmlGetProp() and then discard the result.  
xmlGetProp() returns allocated memory which needs to be freed with 
xmlFree() when you've finished with it.

i also made a 32x32 image of the menueditor and backdrop settings icon, 
and changed the makefile so they go to 
$datadir/icons/hicolor/(size)/apps.  this way someday we can perhaps 
have themed icons for the settings panels.  i'm going to go ahead and do 
the same with the rest of the settings panels sometime in the next day 
or so.

oh, if you update libxfcegui4 and xfdeskop, make sure you kill and 
restart xfce-mcs-manager.  the mcs plugins pull a lot of external 
symbols from xfce-mcs-manager, and the new icon finding function code 
will be missing (and thus the settings panel won't load) until you 
restart the manager.

oh, olivier - i noticed when i killed and restarted the mcs manager that 
xfwm4 didn't pick up the theme change, and thus was stuck with the 
default gtk2 theme for the window menu.  when i change themes via the 
control panel it still doesn't notice.

    -brian