[PointerEvent algorithms] Remove 1:1 correspondence between Mouse and Pointer boundary events

[mustaqahmed]

The UIEvents spec assumes there is "almost" a 1:1 correspondence between mouseenter and pointerenter (and same for other boundary events: mouseleave, mouseover, mouseout). For example, Step 4.1.4 in UIEvents Section 3.4.3.15. handle native mouse move says “maybe send pointerenter event with mouseenter” where the “maybe” is vague and yet it is unable to cover the underlying complexity with multiple pointers.

Possible solution

Solve #519 (Order of boundary events) first, then link the corresponding algorithms in UIEvents and PointerEvents through PE Section 11.1 Tracking the effective position of the legacy mouse pointer.

[mustaqahmed]

FYI: @garykac @flackr @smaug----