removal of the notion of a homogeneous collection from XForms 1.1 WD

During February 28, 2007 telecon the usefulness of the notion of a 
homogeneous collection was discussed. Members agreed to remove this 
notion from XForms 1.1 WD completely cause:

- it's confusing to users
- the definition of continuity of elements is weak as John pointed out 
(existence of text nodes in between elements)
- it is not considered a performance issue by implementors
- it constrains form authors in certain use cases