Access to the event target property

## Feature Request

There is currently no way to access the [`event.target`](https://developer.mozilla.org/en-US/docs/Web/API/Event/target) property (like the clicked element for `onclick` events) from the Rust event handlers. This makes it hard or impossible to do dynamic events based on the clicked elements or events that affect the clicked elements).

This was already requested in #2590, but that issue was closed without a proper solution, as `web_event` does not work for native and mobile targets.

## Implement Suggestion

There should be a way to access (and potentially modify) the event target element of the event.