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.