wayland/data-device: Clear drag source when cancelled from stage grab context
This ensures that applications are notified when a drag gets cancelled because the user dropped or press ESC while in overview.
This fixes an issue with Chromium on Wayland refusing to acknowledge wl_pointer::enter events after accidentally dropping a Chromium-originated object in GNOME Shell overview.
This MR additionally changes the code path used for cancelling the drag on ESC for consistency reasons.