Introduce CallsNetworkWatch to notify of network changes

The libsofia-sip stack needs to bind to a specific interface when there are
multiple network interfaces available.
Handles should be recreated when the default route changes.

src/meson.build

@@ -114,6 +114,7 @@ calls_sources = files(['calls-message-source.c', 'calls-message-source.h',
                        'calls-account-row.c', 'calls-account-row.h',
                        'calls-settings.c', 'calls-settings.h',
                        'calls-secret-store.c', 'calls-secret-store.h',
+                       'calls-network-watch.c', 'calls-network-watch.h',
                       ]) + calls_generated_sources
 
 calls_config_data = config_data