These changes will be used by the next commit when displaying a
conflicting session dialog.
session-removed signal will be used to close the conflicting session dialog
if it's not needed anymore.
getSession method will be used when a session is opened, to check if
there's already a conflicting opened session.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3134>