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/shellMountOperation.js

@@ -398,7 +398,8 @@ const ShellMountPasswordDialog = new Lang.Class({
                          key:    Clutter.Escape
                        },
                        { label: _("Unlock"),
-                         action: Lang.bind(this, this._onUnlockButton)
+                         action: Lang.bind(this, this._onUnlockButton),
+                         default: true
                        }];
 
         this.setButtons(buttons);