No longer allow the Column's 'data' argument to be callable. This was tedious to...

No longer allow the Column's 'data' argument to be callable. This was tedious to use anyway, and only made the declared_name/name/data thing more confusing that it already is. Instead, it is now possible to define render_FOO methods within the table.

Also, these render methods will receive no the BoundRow as an argument, but rather the actual underlining data row. This makes much more sense, and prevents users from having to fall back to the undocument ``BoundRow.data`` attribute to avoid infinite recursion.