NEWS: Mention changes for the next API-adding release.
NEWS: Mention changes for the next API-adding release.

So we remember to mention them. These just missed the 3.16.0 release.

GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm