shellDBus: Add new 'ExtensionStateChanged' signal

The existing 'ExtensionStatusChanged' signal has a fixed set of parameters,
which means we cannot add additional state without an API break.  Deprecate
it in favor of a new 'ExtensionStateChanged' signal which addresses this
issue by taking the full serialized extension as parameter.

https://bugzilla.gnome.org/show_bug.cgi?id=789852

js/ui/extensionSystem.js

@@ -159,15 +159,14 @@ var ExtensionManager = class {
 
         let message = `${error}`;
 
+        extension.error = message;
         extension.state = ExtensionState.ERROR;
         if (!extension.errors)
             extension.errors = [];
         extension.errors.push(message);
 
         log('Extension "%s" had error: %s'.format(uuid, message));
-        this.emit('extension-state-changed', { uuid: uuid,
-                                               error: message,
-                                               state: extension.state });
+        this.emit('extension-state-changed', extension);
     }
 
     loadExtension(extension) {