Description of Pull Request:

In versions of Vue prior to 2.6, scoped slots and un-scoped slots were separate entities. Now all regular names (un-scoped) slots are also available in the $scopedSlots as functions (which provide render speed optimizations, over regular $slots). This PR makes optionally scoped slots work with Vue 2.5.x.

The following slots are now all available as optionally scoped:

• colgroup
• thead-top
• HEAD_<field_key>
• FOOT_<field_key>
• empty
• emptyfiltered
• top-row
• bottom-row

When the user provides a slot-scope, these slots will be called as $scopedSlots, and passed a slot scope context.

Uses the new normalize-slots mixin.

Will also introduce optional scoped slots into other components (such as modal) for passing methods to the slots in a separate PR.

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact:

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e.
  fixes #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR should address only one issue or feature. If adding multiple features or fixing a bug
  and adding a new feature, break them into separate PRs if at all possible.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", etc). This is very important, as the CHANGELOG is generated
  from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and
  event changes)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or
  keyboard only users? clickable items should be in the tab index, etc)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a
  suggestion issue first and wait for approval before working on it)