Update Changelog.txt.

[ci skip]
Keep source file TTML time code format per default
Some minor performance improvements
Formatting.

[ci skip]
Minor refact regarding StringComparison.OrdinalIgnoreCase - thx alphaproject :)
Minor refact
fr-FR translation: restore damage
fr-FR translation: s/deux points/un deux-points/
fr-FR translation: consistently use thin space before [:;!?]