allow to embed a datatable within the parent

As discussed in this thread

http://marc.info/?l=myfaces-dev&m=120118289000330&w=2

add a new attribute to the datatable which allows to "reuse" the layout of the parent table.

The attribute name so far is embed=true|false where false is the default.

When embed=true:

• avoid rendering the table start/end tag
• do not add the thead/tfoot tags around the header or footer - instead simply render them at the start/end of the embedded table
• do not render the header cells using the th tag

Probably we have to think about another way how to define the header on the parent datatable. Not sure about that yet.