Stay out of the way if a game is running
This application is supposed to be friendly and easy-going, so it should stay out of the way if a game is running. We can do this using the com.feralinteractive.GameMode interface. There is a portal for it (https://flatpak.github.io/xdg-desktop-portal/portal-docs.html#gdbus-org.freedesktop.portal.GameMode), although at the moment it does not provide a ListGames method.