Make DnD collector use a DocumentModel instead of a Map

The current implementation has some impediments:

• it's not possible to use complex fields
• it's not possible to use the default values defined in the schemas

Using a document model to store the collected data will address these 2 problems.

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Drag'n Drop Collector screens now make use of a document model instead of a map. This makes it possible to handle complex properties as well as default values in the UI that pops out when doing a drag'n drop.