Subscribe on PreDispatch_Frontend event, so VariantFilter will not apply in backend

[dnoegel]

Should fix the issue with variant filters in combination with product streams. We talked about this fix some while ago, @MarcelSchmaeing

see http://forum.shopware.com/allgemein-f98/problem-mit-gefiltertem-product-stream-t34293.html#p149401

[mitelg]

@dnoegel @MarcelSchmaeing should we merge this?