skins: also flush skin flushing on iconset change
http://bugs.activestate.com/show_bug.cgi?id=100343

they might have caused url changes (e.g.
chrome://icomoon/skin/ -> chrome://global/skin/ ) which will require a flush

(integrated from komodo (trunk) change 81926 by marky)

git-svn-id: http://svn.openkomodo.com/repos/openkomodo/trunk@13629 b87d16e7-d29f-4ee1-b5ec-817d645d335f