[PROPOSAL] Validate TTML subtitles in ISO-BMFF

CCExtractor Version: 0.88

# Necessary information
- Is this a regression (i.e. did it work before)? {NO}
- What platform did you use? {Window}
- What were the used arguments? `{No arguments - we did not attempt to run this type of file through the gui because we did not see an option for the ISO-BMFF.}`

# Additional information
Add a feature to validate TTML subtitles in ISO-BMFF format per specification 14496-30, the format used in DVB Dash and ATSC 3.0).