Don't show confirmation prompt when quitting w/ Command-Q or quit menu item.

[damellis]

In Arduino 1.6.4 (on Mac at least), Command-Q or Arduino > Quit with one window / sketch open brings up the same confirmation prompt you get if you close (with Command-W or the toolbar icon) the window / sketch. (See screenshot.) Since the user is explicitly quitting here, it seems unnecessary to display this confirmation prompt. The prompt doesn't appear if you have more than one sketch open.