Add argument to start initial instance in the background

I'm using PostmarketOS on a Pinephone with phosh, which includes GNOME Clocks. We're working on getting alarms working reliably, across suspends and system restarts. This patch goes part of the way to making that happen. It allows for a "background" mode that initializes the app but simply doesn't show the window. All subsequent instance requests will present the window.

With this patch, someone can create an XDG autostart file that runs gnome-clocks -b, which enables alarms to work on startup.