MimeType selector pattern for textareas

Add mimetype selector pattern for textareas.

The mimetype selection box pattern config options allow to switch between different textarea widgets when changing the mimetype.

Textarea patterns must implement a destroy method, so that the previous widget can be removed when switching mimetypes. This pull-req includes this method for the TinyMCE widget.

Unfortunately, there can only be one mimetype entry per mimetype, so it's not possible to switch between "text/html (TinyMCE)" and "text/html (CKEditor)" or the like.

After merging this, I'll create pull-requests on plone.app.widgets and Products.CMFPlone.