xwayland: Release xdnd grabs ASAP

We currently wait for the selection being cleared by the drag source,
which might not happen on not quite educated clients. This may leave
a stuck XDND grab in the compositor side.

We can actually do a bit better, and clear the grab if:
1) The drag source sent XdndDrop to the wayland drag destination.
2) There's no accepting drag destination and all pointer buttons are
   released.
3) As usual, whenever the drag source clears the selection data

https://bugzilla.gnome.org/show_bug.cgi?id=763246