Merge branch 'dnd-gestures-2' into 'master'

Add new dnd api

See merge request GNOME/gtk!1278

diff --cc docs/reference/gtk/meson.build
Simple merge

diff --cc gtk/gtkcalendar.c
Simple merge

diff --cc gtk/gtkmain.c
Simple merge

diff --cc gtk/gtktreeview.c
Simple merge

diff --cc gtk/meson.build
Simple merge