Add both document and task as properties in task forms

I should be able to use a "document" variable when editing the task form so as to display metadata of the document in my task form. Use case is mostly for reading, so as to let the user have all the information he needs in its task view if he wants.

• Rename “document” to “task” when scaffolding a task form layout
• Support multiple properties in form layouts
• Store these in the view model and update validation and inspection
• Add also a “document” property with dublincore schema when scaffolding a task form