* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

match project's style check

Don't reference `Tether` via attachment to `window`, with the update one can import bootstrap providing the dependencies in webpack with:

```
new webpack.ProvidePlugin({
  $: 'jquery',
  jQuery: 'jquery',
  Tether: 'tether',
});
```

Then inside one's own bootstrap/globals, `import 'bootstrap';` will simply work, and $/jQuery can be used from there.

I had wanted to do this, but also expose jQuery, Tether, etc when in development build in my code, but if I provide `window.Tether`, I can't then expose it to the outside...

Errata from:
* 8ce5da30
* 5d3144e8
which missed a few numbers.

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When a DOM node is passed to an HTML tooltip, the `title` node is only
moved if it is not already in the tooltip. Otherwise, `empty()` is used
instead of `detach()` before appending the `title` to avoid memory
leaks. If a DOM node is passed to a plain text tooltip, its text is
copied via jQuery `.text()`.

Replaces `.detach()` with `.empty()`, as `.detach()` is almost never
useful but instead leaks memory. The difference between `empty` and
`detach` is that the latter keeps all the attached jQuery events/data.
However, since we do not return the previous children, the user would
have to keep these themselves, thus they can `detach()` if necessary.

This is a port of https://github.com/twbs/bootstrap/pull/14552 to v4.

fixes #17097: Go back to using jQuery's text and html methods since innerText is nonstandard and not present in Firefox

Closes #17272 by merging a tweaked version of it.

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