Fix generic type retrieval inside SetPropertyHeaderCommand

The property editing information for a given column (included the DOMELEmentFactory) are set inside SetPropertyHeaderColumn.
If the property is a collection, its generic type is retrieved with informations from the context.
Currently, this mechanism uses the "steps" stored inside "elements", that point to full path of the property (and a mapped entry must exists).
This is broken for top-level collection because are mapped in a different way, so a compatible method for different situations must be implemented