Add support for xtype "transparent"

a new xtype "HST.Transparent" shall be added, which does not insert DOM elements. The Channel Manager shall recognise containers of this type and use a dedicated strategy to determine the DOM elements which represent the container and component(s):

• the DOM element representing the container is the parent of the container metadata comment
• the DOM element representing a component inside a container of x-type transparent is the next (sibling) DOM element of the component metadata comment.

When using this xtype, all components must have a single root DOM element, representing the box into which the component renders, for the Channel Manager to work correctly.