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
nl-NL translation: minor corrections/improvements + new tags
nl-NL translation: spcorr + experiment (buttons)
nl-NL translation: preferences 1