Simplify identifier formatting

Create an IndentifierCol. It'll be used for identifiers instead of IntCol.
It's now possible to do the following with it without having to define a
get_order_number_str:

  >>> model.identifier = 666
  >>> model.identifier
  666
  >>> str(model.identifier)
  '00666'
  >>> unicode(model.identifier)
  u'00666'

So, remove all use cases of order_number/order_number_str/get_order_number_str.
Instead, use identifier/unicode(identifier)/str(identifier)/etc.

Also, create an IdentifierColumn to be used on ObjectList to avoid code
duplication. Some of those IdentifierColumns had a title referencing 'id' or
'number'. Instead use '#' that's the default we use elsewhere.

Change-Id: I74c871c17b6aed45d657aadb478ff628fa8fd843
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5397