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/networkAgent.js

@@ -147,7 +147,7 @@ const NetworkSecretDialog = new Lang.Class({
 
         this._okButton = { label:  _("Connect"),
                            action: Lang.bind(this, this._onOk),
-                           key:    Clutter.KEY_Return,
+                           default: true
                          };
 
         this.setButtons([{ label: _("Cancel"),