Added prev/next navigation support for text, images, video. Fixed Firefox spurious br tag insertion issue.

[cubiclesoft]

This adds improved keyboard navigation "to" support for text, images, and video. That is, navigating to non-text elements. Navigating away "from" images and video is a separate commit.

This also fixes the navigation issue in Firefox for spurious 'br' tags.

Partially fixes some of the issues in https://github.com/GetmeUK/ContentTools/issues/487