unbreak GTK3 and update desktop-file-utils

[Koop Mast]

Unbreak GTK3, the gtk30 port was updated to 3.6 a few months ago.
Switch to USES=desktop-file-utils, the USES does the running of 
update-desktop-database for us.
Break out USES=desktop-file-utils and INSTALLS_ICONS out of the GTK2 & 
GTK3 options so the plist lines don't get added twice in case of both 
GTK2 & GTK3 options are both enabled.

http://people.freebsd.org/~kwm/libreoffice-gtk.diff

-Koop