Horizontal layout box for dialog UI elements, auto-expends to available width of container.
This class is not really part of the API. It just illustrates the properties
that developers can use to define and create horizontal layouts.
Once the dialog is opened, the created element becomes a CKEDITOR.ui.dialog.hbox object and can be accessed with CKEDITOR.dialog.getContentElement.
// There is no constructor for this class, the user just has to define an
// object with the appropriate properties.
widths: [ '25%', '25%', '50%' ],