This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

element

* Remove aria-expanded from collapse.js target element

aria-expanded="true"/aria-expanded="false" only applies to the trigger,
not the element that is being expanded/collapsed.

* Tweak collapse.js accessibility section

...to make it clearer that the aria-expanded attribute always just goes
on the control.

* Fix collapse.js unit tests

- reword some of the text to make it clear we're checking behavior of
trigger/control
- move incorrect aria-expanded out of the <div>s and to the actual
trigger/control <a>s
- fix incorrect test assertion text output false -> true

* Replace element.offet* by getBoundingClientRect()

* Use variable to store BoundingClientRect

* Fix cc issue...

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

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

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