fix: Fix CompositeItem not emitting blur events when unstable_virtual is true

[diegohaz]

If the user clicks on a CompositeItem element and then uses arrow keys to move to another item, the blur event is not fired on the first item.

This PR moves the logic of getting the previousElementRef to the onFocus event on the Composite component.

How to test?

See added test.

Does this PR introduce breaking changes?

No.

[github-actions]

Size Change: -5 B (0%)

Total Size: 257 kB

Filename	Size	Change
packages/reakit/dist/reakit.min.js	35.9 kB	-5 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
packages/reakit-playground/dist/reakit-playground.min.js	187 kB	0 B
packages/reakit-system-bootstrap/dist/reakit-system-bootstrap.min.js	19.1 kB	0 B
packages/reakit-system-palette/dist/reakit-system-palette.min.js	8.85 kB	0 B
packages/reakit-system/dist/reakit-system.min.js	2.45 kB	0 B
packages/reakit-utils/dist/reakit-utils.min.js	3.46 kB	0 B

_{compressed-size-action}

[ariakit-bot]

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[codecov]

Codecov Report

Merging #858 (00e27a6) into master (cd0c4df) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #858   +/-   ##
=======================================
  Coverage   94.88%   94.88%           
=======================================
  Files         231      231           
  Lines        3519     3519           
  Branches      958      959    +1     
=======================================
  Hits         3339     3339           
  Misses        179      179           
  Partials        1        1           

Impacted Files	Coverage Δ
packages/reakit/src/Composite/Composite.ts	100.00% <100.00%> (ø)

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