* Improve code coverage for our Button plugin

* improve coverage for our Alert plugin

* test tooltip update method

* test update and dispose for dropdown

* increase code coverage requirements

Also, add a missing license header in build/workbox.js.

* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

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.