Add support for specifying an optional class name for the dialog box when creating modal popups

By default the modal dialog has the xdialog-box classname, but it should be possible to specify another custom, more specific classname. The "Share by email" dialog needed such a classname, and the JS that displays the dialog had to add it manually by walking the DOM tree up to the dialog box.

Now, an optional classname can be added by passing it under the extraClassName parameter of the optional third constructor argument, the options map.