[Table] Introduce TableBodyCells component to avoid unnecessary cell renders

[cmslewis]

Addresses #1547

Checklist

• Include tests (no need; just an under-the-hood refactor)

Changes proposed in this pull request:

Before:

• TableBody used to invoke props.renderCell for every cell in view, every time selectedRegions changed, just to run a diff against the currently rendered cells. That's a lot of wasted work for an interaction that doesn't affect the cells themselves.

After:

• The TableBodyCells component now encapsulates all cell rendering and doesn't respond to selectedRegions changes. This means that we now do zero work in the TableBody when that prop changes. Woo!
  • This also greatly simplifies our shouldComponentUpdate logic in TableBody.

Reviewers should focus on:

• Look at the React Dev Tools in Chrome, and tick the "Highlight updates" checkbox.
• Click to move the selection around.
• Note that nothing in the table body is highlighted, because nothing in the table body repaints on the selectedRegions change anymore.

Screenshot

[gscshoyru]

To be clear: mostly, all you did was made a different separation of concerns for rendering cells vs rendering regions, right? So that changing regions doesn't re-render cells? (which is a boat load of work)

Looks good to me!

[cmslewis]

FAILED TESTS:
    onCompleteRender
      ✖ triggers immediately on mount/update with RenderMode.BATCH for very small batches

Hmm, will look into this after lunch.

[blueprint-bot]

Fix tests

Preview: documentation | table
Coverage: core | datetime

[llorca]

Preview looks good. All those unnecessary renders cut!

[blueprint-bot]

Merge branch 'master' into cl/table-body-cells-component

Preview: documentation | table
Coverage: core | datetime