Multi-pointer support is missing

[jasongrout]

Description

For the last few years we've started using pointer events for dockpanel, splitpanel, dragdrop, etc. However, we are not paying attention to the pointer id. I think that means there are a lot of places where confusing things happen if we have multiple pointers. For example, it seems that you can have one pointer trigger a pointerdown event, then a different pointer trigger a pointermove event, and the two pointers will get confused.

Reproduce

I haven't tried reproducing an issue yet, just looked through the code, but I imagine on a touchscreen you can try pressing a dockpanel splitter, then with another pointer moving past the splitter. Both pointers will cause pointermove events, so the splitter will keep jumping between them.

Expected behavior

The pointer that initiated a drag-type event is the only one that respects the drag.

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