Assert an X selection of the type SELECTION with and value VALUE.
SELECTION is a symbol, typically `PRIMARY', `SECONDARY', or `CLIPBOARD'.
(Those are literal upper-case symbol names, since that's what X expects.)
VALUE is typically a string, or a cons of two markers, but may be
anything that the functions on `selection-converter-alist' know about.
FRAME should be a frame that should own the selection. If omitted or
nil, it defaults to the selected frame.
On Nextstep, FRAME is unused.
(fn SELECTION VALUE &optional FRAME)