Add API for apps to request permission to run in the background
xdg-desktop-portals offers an API for applications to ask permission to run-in-background (without a window) and for permissions to auto-start. See https://flatpak.github.io/xdg-desktop-portal/portal-docs.html#gdbus-org.freedesktop.portal.Background
Semantically this is part of the lifecycle management of a
GApplication so I think it could be useful and beneficial to provide a GLIB API for apps to run in background.