Note that the return value may be owned by a different
process if a foreign window was previously created for that
window, but a new foreign window will never be created by this call.
Return: if there is a selection owner
for this window, and it is a window known to the current
process, the #GdkWindow that owns the selection, otherwise
%NULL.
Determine the owner of the given selection.
Note that the return value may be owned by a different process if a foreign window was previously created for that window, but a new foreign window will never be created by this call.