Datamanager 2 composite widget

As with all subclasses, the actual initialization is done in the initialize() function, not in the constructor, to allow for error handling.

This widget supports the composite type or any subtype thereof. Each child object will be displayed using its own AJAX form inside a MidCOM style element listing: _dm2_composite_xx_header, _dm2_composite_xx_item and _dm2_composite_xx_footer where xx is the value of the style_element_name.