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.

@thet It appears that there is a benefit for keeping profile registration in the root. It will override/hide the default profile in add-on installation form so that only the correct profile (the bbb one) is visible for Plone 4 users.