macOS: Add "quick-terminal" option to "macos-hidden" config

[mitchellh]

Builds on #4538

Add a quick-terminal enum value to macos-hidden that only hides the macOS app if there are no active terminal windows except the quick terminal. Importantly, I think we should keep the macOS app visible if there is any other active non-terminal window such as an update notification.

This will require some changes to the macOS app, I think we should add a notification from our TerminalManager whenever a terminal window is added/removed that AppDelegate can listen to so that our activation policy is fully managed at the AppDelegate level.