Pointer<Enter> and Pointer<Leave>

[JMS55]

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

We want an event like Pointer<Over> or Pointer<Out> that bubbles from children automatically.

What solution would you like?

Add Pointer<Enter> and Pointer<Leave> 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.