Support invoking radio action handlers on click in config form actions.

Unlike a checkbox, a second click on a radio input won't trigger a
'change' event, since you can't uncheck a radio input without clicking
on a different radio input. This made it impossible to respond to click
events (which I have a need for in another change -- a "Custom" radio
input entry that invokes a dialog).

To get notified on a click event, actions can set
`dispatchOnClick: true` on the action. Clicking the radio or its label
will then result in the action handler being called, even if the radio
is already checked.

Since the browser will fire both a change and a click event, and there's
nothing we can do about that, we need to use debounce() to ensure the
handler is only called once.

Reviewed at https://reviews.reviewboard.org/r/6161/