main: Add support for running commands

Add the ability to run a command when starting gnome-kiosk from the command line, and exit when the command terminates, e.g.:

$ gnome-kiosk -- xterm

That allows to quickly test gnome-kiosk with various clients without resorting to specific scripts.

This feature is already present in mutter since GNOME 42, and therefore this change is similar to the change in mutter from the merge request mutter!1982 (merged).