It may be desirable to use icons for action buttons instead of just text labels.  An example is a music player that wants to have previous and next buttons.  It might be nice to support using icon names from the icon naming spec.

The spec allows sending two strings for each action.

"Each even element in the list (starting at index 0) represents the identifier for the action. Each odd element in the list is the localized string that will be displayed to the user."

We probably need to use the localized string for a11y purposes so I guess we need to use the action name.  I don't think we can do this unconditionally, however, since some clients may use icon names as action identifiers without realizing it.

I suppose we need to

 * add a server capability
 * add a per notification hint that icon names may be used as action ids