When tabbing through the interface and landing the focus on the button, hitting "enter" (or 'space') does not activate the button.

This is even more crucial because of the routine that we're using to change the focus when adding and removing fields. If there are existing fields, we focus the first one, but if there are no existing fields, we focus the 'add all' button -- but then, keyboard actions don't work, which makes the focus senseless and a little frustrating.

This is actually a byproduct of OOUI actions. The button has 'click' listener, but also a keypress/keydown listeners, that are separate. All of those, however, emit 'click' event when an action is taken. Instead of extending onClick action (which only accounts for actual mouse click) we should listen to the internal event "click" and act on that.