Return non-nil if OBJECT is a frame.
t for a termcap frame (a character-only terminal),
'x' for an Emacs frame that is really an X window,
'w32' for an Emacs frame that is a window on MS-Windows display,
'ns' for an Emacs frame on a GNUstep or Macintosh Cocoa display,
'pc' for a direct-write MS-DOS frame.
See also `frame-live-p'. (fn OBJECT)