Disable running in notification area (system tray) / quit when closing the window

This option is very useful when used together with "Open Keybase on startup" disabled.

Slack has this option:

> Leave app running in notification area when the window is closed

Telegram has this option:

> Show tray icon

Skype has this option:

> Upon closing, keep Skype running
> > Keep Skype running after closing the application window.

Start by tracing the desktop window-close behavior and notification-area handling described in the issue. Define the setting so users can choose whether closing the window quits the application or leaves it running, then verify both choices and the interaction with disabled startup launch.

Written by the indexing model from the issue text.