[Table] Fix onSelection being called too much in controlled mode

[cmslewis]

Fixes #1529

Checklist

• Include tests

Changes proposed in this pull request:

• 🐛 FIXED Table no longer invokes onSelection excessively on mousemove when selectedRegions is controlled.

Reviewers should focus on:

• Before, I was comparing the next selectedRegions candidate to this.props.selectedRegions under the assumption that the prop would contain the last emitted value. That obviously doesn't hold up when selectedRegions is controlled.
• Now, I'm keeping a lastEmittedSelectedRegions instance variable in DragSelectable. I compare the current selectedRegions candidate to this old value to determine whether I should invoke onSelection again. If the values are deeply equal, I don't invoke.
  • Does that sound good?

Screenshot

[blueprint-bot]

Fix onSelection being called too much in controlled mode

Preview: documentation | table
Coverage: core | datetime