BREAKING: Remove Button's tooltipGapSize prop

[gnapse]

Short description

The tooltipGapSize button prop was introduced in #636, but was challenged shortly afterwards, as not being a good course of action. See #636 (comment).

However, the reason for needing it was related to the fact that icon-only buttons are too aggressive in assuming they should use the aria-label for the tooltip if no tooltip is given. This PR makes it so that icon-only buttons only fall back to use aria-label as tooltip if the tooltip is really omitted (that is, the prop is not given at all, or given with value undefined). If the tooltip prop is given, but with a falsy/empty value, the implicit tooltip is now omitted, allowing consumers of the button to fully own the rendering of the tooltip, or to really suppress it if they really want to (with great power comes great responsibility).

So, from now on, instead of this:

// Not good
<Button
  icon={<SomeIcon />}
  aria-label="Click me"
  tooltipGapSize={8}
/>

We should use this:

// Good
<Tooltip content="Click me" gapSize={8}>
  <Button
    icon={<SomeIcon />}
    aria-label="Click me"
    tooltip={null}
  />
</Tooltip>

However, the most common use case (not needing to customize the tooltip), remains as simple as ever:

<Button
  icon={<SomeIcon />}
  aria-label="Click me"
/>

Or customizing only the tooltip content, but nothing else:

<Button
  icon={<SomeIcon />}
  aria-label="Click me"
  tooltip="Click me NOW"
/>

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Updated all static build artifacts (npm run build-all)

Versioning

This is a breaking change, which implies a new major version release.