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/misc/gnomeSession.js

@@ -7,7 +7,8 @@ const Signals = imports.signals;
 const PresenceIface = {
     name: 'org.gnome.SessionManager.Presence',
     methods: [{ name: 'SetStatus',
-                inSignature: 'u' }],
+                inSignature: 'u',
+                outSignature: '' }],
     properties: [{ name: 'status',
                    signature: 'u',
                    access: 'readwrite' }],