Unicode lexicon

Remove test_contentchooser_search_tab.robot because its not needed to test catalog searches into a functional test. Instead of that, add support for Products.UnicodeLexicon on Plone < 4.3, this way we have i18n aware searches. Its up to the user to declare the dependency into buildout, collective.cover doesn't force the installation of Products.UnicodeLexicon.

fixes #276 
invalidates #285