More hooks

[AdrienClairembault]

Changes description

Some additionals hooks needed for advancedforms.

Two very simple hooks:

• formcreator_question_description triggered when displaying a question's description
• formcreator_question_default_value triggered when displaying a question's default value

And a more complex hook formcreator_actors_type that allow to define custom actors.

This hooks expect a class name, which must implement the new PluginFormcreatorPluginTarget interface.
You'll find an example here: https://gist.github.com/AdrienClairembault/89346d3e545260651f8375a772fc06e6, which allow to select an actor specified in a question from a previous form of the same workflow.

The only impact on existing code is a rewrite of the changeActor javascript which handle hiding/showing the custom html forms for each actors types.

This javascript code now rely on data attributes instead of specific instructions, which was needed to allow plugins using the formcreator_actors_type hook to have their specific html forms displayed.

Also it seems that the same changeActor was written twice, I've deleted the second occurrence.