treewide: Document changes and release 1.42.0

• New layouts:
  • Belarusian (by)
  • Japanese (jp)
  • Serbian (rs), (rs+latin), (rs+latinunicode)
• Fixes:
  • "Keyboard Settings" in the language-selection-popover will now open the correct panel in GNOME Settings.
• Changes:
  • The documentation has been improved. In particular, it should now be much easier to learn how to test changes to Squeekboard and layouts.
  • After building a Debian-package for Squeekboard and installing it, update-alternatives is now supported.
  • The layout-files now have the English name of the language for which the respective layout is, in a comment at the top of the file.
  • For a few languages which do not have a layout yet, a more appropriate fallback-layout will be chosen.