1) Ensure that any DELETE requests from the target are sent to GDK, even if
both the source and the target are in the same process and it
is therefore possible to use a shortcut and call the handler directly
in GTK layer
2) Ensure that target GDK doesn't do anything when GTK asks it to send
a DELETE request, just report back immediately (the code up the stack
does not check for successfullness when request is DELETE, so not giving
it any data is OK).
The source code already synthesizes a DELETE request, so that side is
also taken care of.
https://bugzilla.gnome.org/show_bug.cgi?id=786509
3fd23fcfd1