Use GtkApplication::register-session property to register

We can use GtkApplication to register with the GNOME session rather
than doing it ourselves, simplifying things in CallsApplication very
slightly and allowing us to remove session.{c,h}.

src/meson.build

@@ -56,7 +56,6 @@ calls_sources = files(['calls-message-source.c', 'calls-message-source.h',
                        'calls-call-record.c', 'calls-call-record.h',
                        'calls-record-store.c', 'calls-record-store.h',
                        'calls-call-record-row.c', 'calls-call-record-row.h',
-                       'session.c', 'session.h',
                       ])
 
 calls_config_data = config_data