Cannot expand tree using just a keyboard

Problem/Motivation

I noticed when trying to use the keyboard and tabbing to the field that uses the widget, I could not navigate to the button.

Proposed resolution

Adding tabindex="0" will allow you to navigate to the button but because it is a div you have to then handle a keypress. I think we can bind to the "Enter" key which is similar to how fieldsets expand. Ideally the button is not a div but that is a larger problem to fix.

I will submit a patch with the tabindex and keypress event handler as the suggested approach.