sato-icon-theme: use gtk-icon-cache helper class

sato-icon-theme: use gtk-icon-cache helper class

Instead of explicitly updating the icon cache use the helper class that also
forces a loader update at the same time.  This eliminates the possibility of
updating the icon cache without any gdk-pixbuf loaders.

Also check that the Sato icon theme isn't already set to avoid appending to the
file every time the postinst runs.

[YOCTO #2399]

(From OE-Core rev: 9d98dbdae4c05fcf50d546f554a04dc3f0bd66c3)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>