sip: Make save_to_disk() public and use it when updating accounts

Account credentials will not get updated at all otherwise.

plugins/sip/calls-sip-account-widget.c

@@ -400,6 +400,7 @@ on_apply_clicked (CallsSipAccountWidget *self)
                                     TRUE);
 
   update_header (self);
+  calls_sip_provider_save_accounts_to_disk (self->provider);
   g_signal_emit_by_name (self->provider, "widget-edit-done");
 }