Pointer<Enter> and Pointer<Leave>

[JMS55]

What problem does this solve or what need does it fill?

We want an event like Pointer or Pointer that bubbles from children automatically.

What solution would you like?

Add Pointer and Pointer events.

Additional context

See https://developer.mozilla.org/en-US/docs/Web/API/Element/mouseleave_event.

[NthTensor]

This is going to have to involve traversing the hierarchy a bit within the picking event dispatch system. I think I have stuff for this laying around somewhere, should be able to get it in for 0.16.

In the event that I dont':

• We need to add a within: HashMap<Entity, HashSet<Entity>> field to PointerButtonState.
• At the start of pointer_events we create a new entity set now_within and add every hovered entity to it along with their ancestors.
• Between Out and Over we send a Leave event to everything in within but not now_within and an Enter event to everything within now_within but not within. These events should not bubble. Then we set within equal to now_within.

We may want to consider changing DragEnter/DragLeave to run as part of Enter/Leave rather than Over/Out. We may even want to consider removing Over/Out.