Enable navigating grid headers by tab-key

[TomasLongo]

Description

Make grid headers navigable via tab keys

Issues Resolved

Resolves: #1501

Check List

• New functionality includes testing.
• New functionality has been documented.
• All tests pass
  • yarn lint
  • yarn test-unit
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

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[TomasLongo]

Hey everyone.

I'm new to the project and also not an expert in UI development. So, chances are, that my changes do not take into account possible impacts on other parts of the ui.

The change in it's current form is simple. but most likely not complete. The code surrounding those changes makes me wonder:

• Why have header cells been taken out of the tab index at all? Especially, because the same function, while setting tabIndex to -1 also sets an attribute data-ouigrid-tab-managed to true. Seen from my current position, this contradicts the removal from the tab index (this conclusino is most certainly wrong)

To be complete, ControlHeaders should also be tabbable. However, I wasn't able to make it work just like I managed to do it with the normal headers. This might be done in a separate step.