FeatureFormView: Add "close on save" mode

[rolson]

Can we add a mode so that when the user hit's save it also closes? Could be exposed as a view modifier to enable.

[dfeinzimer]

The developer may not provide the FeatureFormView the ability to dismiss itself if they don't use the isPresented initializer parameter.

The recommended approach would be for the developer to watch for FeatureFormView.EditingEvent.savedEdits(willNavigate:) with onFormEditingEvent(perform:) as a trigger to dismiss the form.