MacOS exit to tray

[zackslash]

This MR aims to provide a more native/intuitive behaviour in MacOS.

• Enables 'exit to tray' by default in MacOS so music can continue in the background.
• Fixes the 'Quit' menu bar item so that the app can be fully quit when in 'exit to tray' mode.
• Fixes the app badge 'Quit' item so app can be fully quit from badge in 'exit to tray' mode.

Demonstration of fix: all settings set to default, app running in tray, opening and closing from system menus:

pr1.mov

Closes #264

[jeffvli]

Looks good. I tested the app.exit() tray functionality on Windows as well and doesn't seem to break anything. Thanks.