Journalize all changes to `public_trial` field

According to the [specification](https://my.teamraum.com/workspaces/onegov-gever-innovation-session/improvement-proposals/ogip6-umsetzung-offentlichkeitsgesetz/view), any change made to the `public_trial` field needs to be explicitely journalized.

This PR implements this requirement:

- Any changes to the `public_trial` fields create a journal entry on the respective document, indicating the new field value.
- The regular journal entries on the document and its parent (dossier) that indicate that the document's metadata has been changed still get created.
- The journal entries get created if editing the field via the regular `edit` form as well as if the change is made in the `edit_public_trial` form for closed dossiers.

The `edit_public_trial` form previously implemented in #380 didn't fire any events when modifying the object. Therefore I changed this form to inherit from `plone.directives.dexterity.EditForm`, so all necessary events get fired, `handleApply` doesn't need to be implemented again, and the journal event subscribers "just work".

@jone could you give this a review please?
I just realized that this is probably in the title `Message` object...