[structure] Try to infer schema type from filter in document lists

[rexxars]

As outlined in #1633, the following structure does not get a schema type set:

S.documentList()
  .title('Unspecified books list')
  .menuItems(S.documentTypeList('book').getMenuItems())
  .filter('_type == $type')
  .params({type: 'book'})

While this shouldn't normally cause any problems, in this case we can actually infer it based on the filter/parameter, which saves us from having to resolve it further down the stack.

[thomax]

LGTM!