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 Good point.

Johannes Raggam wrote:
>
> In setup.py:
>
> > @@ -49,7 +49,7 @@
> >          'plone.app.contentlisting',
> >          'plone.app.dexterity',
> >          'plone.app.layout',
> > -        'plone.app.portlets >= 3.0',
> > +        'plone.app.portlets',
>
> 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.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/plone/plone.app.event/pull/163/files#r14394198>.