extensionUtils: Use signals rather than callbacks for finding extensions

This allows us to move to a file-monitor based approach in the future.

Since we need signals, we convert the current set of functions to an
object we attach signals too, leading to the new ExtensionFinder object.

https://bugzilla.gnome.org/show_bug.cgi?id=677586

js/ui/extensionSystem.js

@@ -262,8 +262,10 @@ function init() {
 }
 
 function loadExtensions() {
-    ExtensionUtils.scanExtensions(function(uuid, dir, type) {
+    let finder = new ExtensionUtils.ExtensionFinder();
+    finder.connect('extension-found', function(signals, uuid, dir, type) {
         let enabled = enabledExtensions.indexOf(uuid) != -1;
         loadExtension(dir, type, enabled);
     });
+    finder.scanExtensions();
 }