Function should_enable 

pub fn should_enable(tray: bool, background_service: bool) -> bool

Should the tray be started at all?

When the tray is on (app:showtray, on by default), or whenever background-service mode is on: a resident process must always show an icon, so background mode forces it. Without background mode the icon simply lives as long as the app does: closing the last window quits, and the icon goes with the process.