Add an 'active' property to ShellButtonBox. This allows ShellButtonBox
to be used as a "toggle button". It's up the application to connect
it to the ::activate signal; there's no default handling of this.

(It's seldom that the only time you want to toggle a toggle button
through the user interface, so you need some connection to the backend
data store in any case. Removing the default handling all-together
prevents weird interactions.)

When we have built-in styling for ShellButtonBox the 'active' state
would be one of the elements that would be affect the styling.