In XDND, we just get a hint on XdndPosition about what's the action
chosen by the user. Make the data source actions the full set on
XdndEnter (as we can't know better), and pass the hint in XdndPosition
as the user chosen action as it should be.
Makes Wayland drop sites aware of the user action as per XDND with X11
drag sources, and still makes modifiers during DnD work.
https://gitlab.gnome.org/GNOME/mutter/issues/974https://gitlab.gnome.org/GNOME/mutter/merge_requests/1005