4.16.0

- settings: Accept all types of GIcon
- settings: Adjust inline toolbar
- Fix compilation with -Wmissing-prototypes
- Don't warn when there are no saved sessions
- bump glib (and gio, gthread, etc) minimum to 2.50.0
- Remove GSourceFunc casts
- Update .gitignore
- Add README.md to EXTRA_DIST
- Translation Updates:
  Basque, Chinese (China), Czech, Danish, Estonian, French, Galician,
  Hebrew, Hindi, Indonesian, Italian, Lithuanian, Norwegian Bokmål,
  Portuguese, Portuguese (Brazil), Russian, Serbian, Slovenian,
  Turkish, Vietnamese