Change GdkDrag::action to GdkDrag::selected-action, which is
more clearly different from actions, and follows the existing
name of the struct field and getter.
This lets us drop the ::action-changed signal for the
property change notification. But, can just as well move
the signal class handers which just update the cursor
to the ::action setter. No need to do this in the backends.
Rename gdkdnd.h to gdkdrag.h, to go along with gdkdrop.h
This commit includes the necessary updates to the X11, Wayland
and Broadway backends. Other backends have to be updated separately.
