Implement #1584 - Fields now registered by their JSON type name, allowing new custom fields and overriding of the standard fields. Replaces the manual switch statement for loading fields from JSON block definitions.
goog.date.fromIsoString has been deprecated in favor of goog.date.DateTime.fromIsoString.
This change still needs to be tested with field_date enabled in the build before submitting.
* New widget div functions, used in context menu code
* Make all widget div positioning functions use the same argument order
* Use new widget div functions for fields
* share code for measuring menu size
* Get rid of positionMenu
* Update copyright date
* Rebuild blockly_uncompressed because there's a new require in town
