bpo-45296: Clarify close, quit, and exit in IDLE (GH-30936)

bpo-45296: Clarify close, quit, and exit in IDLE (GH-30936)

In the File menu, 'Close' and 'Exit' are now 'Close Window' (the current
one) and 'Exit' is now 'Exit IDLE' (by closing all windows).
In Shell, 'quit()' and 'exit()' mean 'close Shell'.
If there are no other windows, this also exits IDLE.
(cherry picked from commit fcde0bc10ddd836b62d0a8e893d80b8c55e0ba3f)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>