Fix: "File > Load Subtitle" wasn't working anymore for DVD.

The unwanted check that limited that menu entry to file playback was mistakenly added in a84038a34eb505fff671956282f8746811e7bbe9.

This commit fixes #3023.
Sanify some unclear code.
Cosmetics: Use explicitly NULL/FALSE constants to make the code clearer.
typo
warnings fix WIP
cosmetics
reduce variable scope
update Changelog.txt
Change the Donations url to the new one.