SLTev format should be playable in online-text-flow and new subtitler

[obo]

This issue should be resolved not here, but rather in our less-public one: https://github.com/ELITR/SLTev-data-converters

We need to explain how to play:

• golden transcripts
• reference translations
• system outputs in online-text-flow by Ota and also in new Subtitler by David Javorsky

This issue is for Mohammad and Ebrahim to work on but David + Ota (or someone else) to test it with their presentation tools.