Return text selected from some X window.
SELECTION-SYMBOL is typically `PRIMARY', `SECONDARY', or `CLIPBOARD'.
(Those are literal upper-case symbol names, since that's what X expects.)
TARGET-TYPE is the type of data desired, typically `STRING'.
TIME-STAMP is the time to use in the XConvertSelection call for foreign
selections. If omitted, defaults to the time for the last event.
TERMINAL should be a terminal object or a frame specifying the X
server to query. If omitted or nil, that stands for the selected
frame's display, or the first available X display.
On Nextstep, TIME-STAMP and TERMINAL are unused.
(fn SELECTION-SYMBOL TARGET-TYPE &optional TIME-STAMP TERMINAL)