[SQLObject] default sort field

Hey again -

I've got a few suggestions, which I'm in the midst of implementing, and curious
whether people 1) see a need and 2) have alternate implementations for these.

Proposal:

Each SQLObject-derived class should be able to set a default ordering of the
results returned by .select()s, and (more importantly?) when accessed via a join.

Our need:

When retrieving objects for display, we often have one primary way that the list
should be viewed. This may be a natural field, such as alpha by name, or it may
be a field explicitly for client-defined order. This requires every request to
remember to order by this field, and makes control of the default order
de-centralized.

Proposed solution:

There's two possibilities: a column could say "I'm default order!", or it could
be a new class-level attribute like "_defaultOrderField = 'name'". We also need
to provide a mechanism to specify ascending or descending order.

We are implementing the first option, with two new class-level attributes:

  _defaultOrderField: must be a field defined in columns, or else it will be
reset to None. By default, None.

  _defaultOrderDirection: less likely to be used, either "DESC" or "ASC"
(case-insensitive). Default is 'ASC', as in SQL 'standard'.

This then requires modifications to .select, and to *Joins .performJoin methods
(as joins do not use .select). If _defaultOrderField is None, the SQL is
unchanged from current.

Thoughts?

- Luke