Add CLI flag to start app minimized

[E1int]

No description provided.

[dweymouth]

What is the use case of this? And how should it behave if the system tray is not enabled? It seems to me that in that case it would start the app with no way to actually show the window.

[E1int]

My use case would be to use supersonic without its GUI. The next part would be adding other commands to the CLI (e.g. playing a playlist) to make this actually usable.

On the tray part, I'm on KDE and saw an icon on the task manager while testing this and thought it would allow to open the main window, but I was wrong. Currently, it would be more correct calling this a headless mode more than a minimized start.

[dweymouth]

I'll be happy to merge this if you can add a --show flag that sends an IPC message to the running instance to make its window visible. You can take a look at the MPRIS handler for an example of how to implement it because the MPRIS handler can respond to a "raise" request which shows the window.

Adding this, a Supersonic instance started with --start-minimized wouldn't be "stuck" being a headless instance

[E1int]

Should be ready to merge.

[dweymouth]

Looks good, thanks!