Fixed check dependency on plone.namedfile

I found an uncommon situation on Plone 4.2.

If the `plone.dexterity` (but **not plone.app.dexterity**) is present in the buildout, TinyMCE is not working because the `HAS_DEXTERITY` check is only looking for `plone.dexterity`, the using also `plone.namedfile`.
This can happen when add-ons (like collective.lorepipsum) are installing plone.dexterity as dependency.

Until dexterity will be part of Plone core, I think that this dependency must be changed as in this pull request (or changing the check on the whole `plone.app.dexterity`.

Probably the same fix must be ported on the master/TinyMCE 1.3.x