extensionDownloader: Add update/blacklist support for extensions

This is a bare-bones copy/replace. It does not implement ChangeLog
support. If we cannot get System Updates integration, I will implement
notification support.

https://bugzilla.gnome.org/show_bug.cgi?id=679099

js/ui/shellDBus.js

@@ -214,6 +214,8 @@ const GnomeShellExtensionsIface = <interface name="org.gnome.Shell.Extensions">
 <method name="ReloadExtension">
     <arg type="s" direction="in" name="uuid"/>
 </method>
+<method name="CheckForUpdates">
+</method>
 <property name="ShellVersion" type="s" access="read" />
 </interface>;
 
@@ -306,6 +308,10 @@ const GnomeShellExtensions = new Lang.Class({
         ExtensionSystem.loadExtension(uuid);
     },
 
+    CheckForUpdates: function() {
+        ExtensionDownloader.checkForUpdates();
+    },
+
     ShellVersion: Config.PACKAGE_VERSION,
 
     _extensionStateChanged: function(_, newState) {