Mark gtk_drag_icon_create_widget_for_value with
GDK_AVAILABLE_IN_ALL, since it is meant to be public.
Also, clarify the documentation a bit.
Fixes: #3325
Before, gtk_drag_icon_new_for_drag() allowed creating new drag icons.
This could cause multiple drag icons to exist for a single drag.
Now, gtk_drag_icon_get_for_drag() makes sure that only one drag icon is
created.