icon cache scriplet guideline update

[Rex Dieter]

FYI,
The packaging committee passed a new guidelines wrt icon cache scriptlets:
http://fedoraproject.org/wiki/PackagingDrafts/ScriptletSnippets/iconcache
(pending FESCo/Core-cabal ratification).

In short, replace existing scriptlets
touch --no-create %{_datadir}/icons/hicolor || :
%{_bindir}/gtk-update-icon-cache --quiet %{_datadir}/icons/hicolor || :
with
%{_bindir}/xdg-icon-resource forceupdate --theme hicolor  || :

-- Rex