`U.SCHEMA` now contains an `impacts` key, which makes it possible to
specify what part of the UI is impacted by data changes.
A new `render` method has been added on `U.Map` and `U.DataLayer`, which
is used to rerender the proper parts of the UI depending on the passed
properties.
`U.FormBuilder` calls this `render()` method (if present), during form
changes.
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