[EuiAccordion] Allow external control

[thompsongl]

Summary

Resolves #3594 by allowing forceState and onToggle to work together as an external state value and update function pair.

When forceState is in use onToggle will be called with a boolean reflecting the current open/closed state. This might be slightly confusing as standard onToggle is called with a boolean reflecting the new open/closed state after click.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox

• Props have proper autodocs
• Added documentation
• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately

- [ ] Checked for accessibility including keyboard-only and screenreader modes

• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3614/

[chandlerprall]

Changes look good, though thinking about

onToggle will be called with a boolean reflecting the current open/closed state. This might be slightly confusing

Thoughts on calling onToggle with the user-initiated state value instead?

[thompsongl]

Thoughts on calling onToggle with the user-initiated state value instead?

I could definitely go either way on this.
My read is that the consumer won't be attempting a bunch of logic around this value and will just update state accordingly, so perhaps passing the state expected by the user after click makes more sense.

[chandlerprall]

so perhaps passing the state expected by the user after click makes more sense.

I think so; let's go that way, then this is good to merge

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3614/

[chandlerprall]

Looks like the accordion_forceState.js example wasn't updated and expects the inverse value in onToggle

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3614/

[chandlerprall]

PUPPETEER_SKIP_CHROMIUM_DOWNLOAD

jenkins test this

[chandlerprall]

Changes LGTM! Tested in the PR's docs

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3614/