Extend QueryBuilder to support mappings of entity attribute name to a differently named column

[muhrin]

At the moment the way that querybuilder works is that it takes filters and assumes that the name maps directly to a column of the same name. This is not the case in, notably, Group where the type_string attribute is actually stored under a column called type.

The most trivial solution would be just to rename the column type_string in a migration. However a better solution would be to have a backend implementation dependent get_column method that supports mapping the filter key to the correct column in the model. This would, however, require that the concrete QueryBuilder gives the get_column more information, specifically at least the orm class Entity type.

For now, I'm just changing the relevant parts of the code (for Group) to use type instead of type_string.