Expose screencast functionality via DBus

Like screenshots, the screen recorder can be a useful tool in other cases
than being triggered by a keyboard shortcut. To account for that, export
a Screencast DBus API similar to the existing Screenshot interface.

https://bugzilla.gnome.org/show_bug.cgi?id=696247

js/ui/shellDBus.js

@@ -12,6 +12,7 @@ const ExtensionDownloader = imports.ui.extensionDownloader;
 const ExtensionUtils = imports.misc.extensionUtils;
 const Hash = imports.misc.hash;
 const Main = imports.ui.main;
+const Screencast = imports.ui.screencast;
 const Screenshot = imports.ui.screenshot;
 
 const GnomeShellIface = <interface name="org.gnome.Shell">
@@ -70,6 +71,7 @@ const GnomeShell = new Lang.Class({
         this._dbusImpl.export(Gio.DBus.session, '/org/gnome/Shell');
 
         this._extensionsService = new GnomeShellExtensions();
+        this._screencastService = new Screencast.ScreencastService();
         this._screenshotService = new Screenshot.ScreenshotService();
 
         this._grabbedAccelerators = new Hash.Map();