fix(xychart): PointerEvent fixes

[williaster]

🐛 Bug Fix

This fixes a couple of issues with PointerEvents in @visx/xychart:

• *Series subscribe to events from EventProvider, but when sources were added there was a mismatch between the emitter source <> handler source, so handlers were not being invoked.

• TooltipProvider had an issue where hideTooltip was debounced, which meant a call sequence of showTooltip => hideTooltip => showTooltip could result in the debounced hideTooltip call being invoked after the showTooltip call and closing the tooltip. This was fixed by canceling the debounced call in the showTooltip handler.

@kristw @hshoff

[coveralls]

Pull Request Test Coverage Report for Build 399422746

• 9 of 13 (69.23%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 61.379%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-xychart/src/providers/TooltipProvider.tsx	3	7	42.86%

Totals
Change from base Build 399258596:	0.007%
Covered Lines:	1720
Relevant Lines:	2633

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💛 - Coveralls