Restore Plone 4.3. support

Do not merge yet.

@thet Almost there. I opened this pull for discussion.

All tests run on Plone 4.3 with correct other versions. I should document them soon. Plone 4.3 compat portlet templates are still missing.

@vangheem Thanks for correcting me on that (and sorry @thet). I guess, I had bad luck with Mr. Roboto and jenkins.plone not getting that version.