Implement tag/model/add in client
Refactor and streamline LabelRequestCallback class

I broke out several smaller helper methods from the chunk of code that
was the onResponseReceived method.
Add static labels to DataTransfer

I use these labels in case statements in event handlers. This will help
preclude spelling errors.
Split handler code for models/new and models/edit
Refactor and streamline ModelRequestCallback class

I broke out a set of helper methods to simplify the onResponseReceived
method.