Simplified QuantityFieldWidget updating
Removed unused FormDesignDialog
Updated FormDesigner to use the TypeRegistry as a source

Notes:
* For naming, let's stick to the convention of using the name 'Field' to describe
  the logical definition of a value in a Form, and 'FieldWidget', or 'Widget' to
  describe the corresponding UI elements.

  Renamed ControlBucket => FieldPalette

* Removed FormViewModel: adds just another level of complexity. Instead, refactored
  the FormFieldWidgetFactory.create() and FormFieldWidget.setValue() methods
  to be asynchronous.