4.16.1

- Add 4.16 section to docs
- about: Replace stock with regular button
- about: Make Close button translatable (Fixes #41)
- Fix cast alignment warning
- Remove Gtk2 leftovers
- Don't reserve vertical space for subtitles in headerbars
- Translation Updates:
  Basque, Belarusian, Bulgarian, Czech, Danish, Estonian, Finnish,
  French, Hebrew, Indonesian, Interlingue, Japanese, Lithuanian,
  Norwegian Bokmål, Polish, Portuguese, Portuguese (Brazil), Russian,
  Serbian, Slovenian, Spanish, Swedish, Turkish