Add content element to create Modals

It would be great to have a content element to create Modals!
It should then be possible to add various content elements into the Modal, like tabs, carousels etc.

No files or tests are named; start by locating the modal implementation and content-element definitions in the repository. Clarify how nested elements such as tabs and carousels should be selected and rendered, then verify that a modal can contain varied content elements as requested.

Written by the indexing model from the issue text.