TokenizingTextBox cannot be modified programatically

[michael-hawker]

Describe the bug

It's not possible to programatically set the existing tokenized items in the TokenizingTextBox.

Expected behavior

Should be able to provide a collection of strongly-typed items to the textbox to have pre-tokenized for form hydration or add/remove individual items later from other sources.

Additional context

This was the 6.0 ship blocker, and deferred the control to 6.1.