Demos: Popup: Removed weird (and almost certainly accidental) popup-inside-popup embedding.
Tests: Popup: Partitioned tests into unit and integration.
Tests: Integration: Popup: Add test to make sure popup elements do not have focus after the popup is closed. Tests #5974.
Popup: Blur any focused elements inside container, including the container itself, upon closing. Fixes #5974 - keyboard does not disappear when after the pop-up is closed.