ModalDialog: add 'default' pseudo-class to default button

Add 'default' parameter to setButtons, that controls the binding
of Return (unless overridden) and applies the 'default' pseudo-class.
Currently it has no effect, but it will start having after the
login dialog redesign.

https://bugzilla.gnome.org/show_bug.cgi?id=619955

js/ui/extensionDownloader.js

@@ -197,7 +197,8 @@ const InstallExtensionDialog = new Lang.Class({
                            key:    Clutter.Escape
                          },
                          { label:  _("Install"),
-                           action: Lang.bind(this, this._onInstallButtonPressed)
+                           action: Lang.bind(this, this._onInstallButtonPressed),
+                           default: true
                          }]);
 
         let message = _("Download and install '%s' from extensions.gnome.org?").format(info.name);