eventHandlers should be a mapping of generic callables

The [`eventHandlers`](https://github.com/idom-team/idom/blob/b7e729247b622a390f08fb8f1cc1bf23aa9ecad6/src/idom/core/vdom.py#L101) key should map to generic callables. Someone implementing a layout shouldn't have to support the `EventHandler` class if they don't want to.