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/status/power.js

@@ -49,7 +49,7 @@ const PowerManagerInterface = {
         { name: 'GetPrimaryDevice', inSignature: '', outSignature: '(susbut)' },
         ],
     signals: [
-        { name: 'Changed', outSignature: '' },
+        { name: 'Changed', inSignature: '' },
         ],
     properties: [
         { name: 'Icon', signature: 's', access: 'read' },