Do not delete the MULTIPLE property after reading it

Do not delete the MULTIPLE property after reading it

Per the ICCCM spec:

> The requestor should delete [...] the property specified in the
> MULTIPLE request when it has copied all the data.

We are not the requestor, so we should not be deleting this property
(which is what x_get_window_property_as_lisp_data does).  The property
needs to remain available as the requestor will generally want to read
it back to see which conversions succeeded or not.

* src/xselect.c (x_get_window_property_as_lisp_data): Add flag which
skips deleting the read property, or handling INCR (which does not
make sense for MULTIPLE).
(x_handle_selection_request): Enable the flag.