Allow complex sortable columns

When attempting to sort by one of my columns I was getting the error
> SQLSTATE[23000]: Integrity constraint violation: 1052 Column 'weight' in order clause is ambiguous

This was due to my *weight* column having a `select: fooTable.weight` property but it not being reflected in the `order by` clause of the SQL being generated by the Lists widget.

This PR allows for columns that use a custom `select` to be sortable.