Ensure that all signatures are defined in DBus interfaces

The DBus JS binding will complain if the signature of a method
or of a signal is undefined, so we need to define it even if it
is an empty string, and we need to use the correct property name
for signals.

https://bugzilla.gnome.org/show_bug.cgi?id=643374

js/ui/endSessionDialog.js

@@ -54,7 +54,7 @@ const EndSessionDialogIface = {
               }
              ],
     signals: [{ name: 'Canceled',
-                outSignature: '',
+                inSignature: '',
               }],
     properties: []
 };