Add "mp4-created" option to set video-time-start/end using mp4 metadata

[jburnhams]

Contributor Checklist

• Review Contribution Guidelines
• Agree to the terms of Developer Certificate of Origin.
• Make sure all contributed code can be distributed under the terms of the GNU Licence
• Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by maintainers if required.
• Unit tests created/updated (under tests) to verify logic, and approval tests for UI Widgets
• Update Examples - Under build-scripts/examples, and regenerate examples docs if required ( make doc-examples )
• Ensure that tests pass locally - this can be tricky for approval tests with maps or text though.
• Check that pre-release tests work ( make test-distribution )

[jburnhams]

I've got a mismatch from timezone / BST - investigating....
Ah no, my mp4 file has the metadata timestamp as UTC when it should be BST. I'll hack it for me but don't think anything needs changing here for it.