Doc webdav: enhance the property notation structure

Doc webdav: enhance the property notation structure

When DAV-aware nodes want to return properties, they might need to
have xml attributes or single instances of sub-elements.

Example:
	return ('aprop', 'D:', ('foo', 'D:', None, { 'name': 'bar'}))
will result in xml:
	<D:prop>
	    <D:aprop><D:foo name="bar"/></D:aprop>
	</D:prop>

bzr revid: p_christ@hol.gr-20101012104029-gq6hhjki4i52n8qh