Select a viewport position and return the window there. Use a special cursor and let the user click to select a viewport position Returns a list of three items: (selected-window viewport-x viewport-y). selected-window is either the window object corresponding to the selected window, a window id as an integer, or #f if no window was selected. viewport-x and viewport-y give the position in the viewport that the cursor was located when the selection was finalized. release? indicates whether to whether to wait for a mouse release or act immediately on the click. cursor is the cursor object to use, or #t for the "skull and cross-bones" kill cursor (recommended for destructive operations like delete-window and destroy-window), or #f or omitted for the standard circle cursor.