[Feature Request] Emit event when v-app-bar reacts to any of the scroll props

[ferferga]

Problem to solve

Developers might need to add further animations or use their own hooks when v-app-bar component reacts to any of the props that react on scroll are used (hide-on-scroll, elevate-on-scroll, etc... )

Proposed solution

As the scroll threshold the app bar component uses is not always the same, it would be great to have an event emitted by the component, so users can sync animations or their own hooks in a predictable way.