- Fixed bug whereby nesting a label of a select()

- Fixed bug whereby nesting a label of a select()
with another label in it would produce incorrect
exported columns.   Among other things this would
break an ORM column_property() mapping against
another column_property().  [ticket:2167].
Also in 0.6.8
- _Label() is always against a column or selectable.  remove
uncovered case of label against something else.
- start taking notes to clean up some of this labeling stuff,
which will be [ticket:2168]