Support dexterity

This pull request adds dexterity support. Before this, the 'create content from template' action was never visible on dexterity folders, and dexterity items could not be used as templates, because copying would fail.

One thing that is not supported on dexterity, is rename after the first edit.  This only works on Archetypes because Archetypes needs special handling as it does not have a concept of a true add-form.

If dexterity is available in `bin/test`, the existing tests are run a second time with dexterity, except for one test that checks the renaming.

I have chosen to only add dexterity to `bin/test` in the Plone 4.3 buildout.  It could be added in `setup.py` instead.  But for 4.1 no dexterity versions are pinned, so that would cause problems.  And on 4.2 there are problems in running the tests with dexterity. Running the tests only on 4.3 might be good enough. In the current setup, there are no failures in 4.1 and 4.2. Any dexterity imports are done conditionally.