Add `hovered` to `on_release()`

[TheRedDeveloper]

Summary

Add a hovered parameter to the on_release() callback to indicate whether the mouse is still hovering over the button when it is released.

Motivation

There is no way to know if the mouse is still hovering over the button when on_release() is called. From #55.

Proposed API

on_release(|id, pointer, hovered| {})

Behavior

• The hovered parameter only will be true if
  • the mouse is hovering over the element
  • it's still focused (keyboard navigation)