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.

we need at least plone.app.portlets ``2.5.1``. With plone.app.portlets below 2.5.1 we get zcml configuration errors, because the calendar and event portlets from plone.app.portlets and plone.app.event are registered under the same name.