Support getImmediatelyAddableTypes for dexterity containers.

We use the ISelectableConstrainTypes adapter from CMFPlone to get the immediately addable types.
This is the first step towards supporting dexterity.
Do not rename copied dexterity items.

Dexterity items have no markCreationFlag or generateUniqueId methods
because they do not need it normally: only Archetypes needs it
because it has no concept of an add-form.
Downside is that dexterity items now simply get the same id
(or prepended with copy_of_) of their original template.
types/Folder.xml: do not claim a meta_type.

For Archetypes it is 'Factory-based Type Information with dynamic views',
but it may be a dexterity Folder, for example from plone.app.contenttypes,
in which case the meta_type is 'Dexterity FTI'.
The previous meta_type did not seem to have a bad effect, but it is unneeded.
Add plone.app.dexterity to test eggs on Plone 4.3.

We may want to do this in setup.py instead.
But dexterity versions are not pinned in Plone 4.1, which we apparently still want to support.
So for the moment we do it like this.
Run the tests a second time with dexterity setup.
Add changelog entry for dexterity update.
Merge pull request #6 from zestsoftware/maurits-dexterity

Support dexterity