4.16.0

- Drop autogenerated xfce-backdrop-settings.desktop.in from git
- Set new wallpaper as default
- Add wallpaper for 4.16
- settings: Drop subtitle
- Remove GLIB_CHECK_VERSION IFDEFs which are obsolete after glib bump
- bump glib (and gio, gthread, etc) minimum to 2.50.0
- Remove GSourceFunc casts
- Avoid warning, pointer target in assignment differ in signedness
- Translation Updates:
  Chinese (China), Danish, Greek, Norwegian Nynorsk, Slovenian