2007/04/12 10:40:30 fs 1.9.322.1: #i76119# remove the constraint that extensible groups cannot have children of type "prop"

The comment for this constraint claims this is an unsupported feature, but this is
not the case:
- the parser at runtime perfectly recognizes it, and properly delivers the respective
  node
- the API itself also recognizes that the node is non-valitile (which is implied
  by it being part of the schema), and rejects any modifications to it.

According to JB, the original implementor, this implies that this construct is
in fact supported.

Note: Other similar restrictions are still intact: An extensible group still
cannot contain any "set" or "group" child nodes.