Add JS for icon property in button, button group & menu button widget

Editor assessment

The subject states the addition concretely: JavaScript bindings for the icon property across button, button group, and menu button widgets. The fix type is arguable, since enabling JS mode on a property leans toward a feature, but straddling types should not dominate the judgment. No scope is present; the subject enumerates the widgets instead. There is no body, acceptable for a small property-enablement change. No footers appear. The message is understandable without the diff and names every affected component, which is the main communication job done well.