add possibility of calculating how many rows there were before LIMIT was applied

The Finder has get_rows_counter() method which returns how many rows there were
before LIMIT clause was applied. The select(...) method should be called with
Select::with_rows_counter flag in such a case.

while here:
- change the semantic of Finder, now the select(...) method takes a morm::Select flags,
  and we have such flags:
  - Select::no_auto_generated_columns - do not generate columns from models
  - with_rows_counter - add an additional column for the rows counter
- remove Finder::prepare_to_select() - now use select(...) with no_auto_generated_columns flag

src/baseexpression.h

@@ -334,6 +334,7 @@ protected:
 
 	virtual void save_foreign_key(const wchar_t * field_name, const FT & field_type, ModelEnv * model_env);
 	virtual void dump_additional_info(Model & model);
+	virtual void add_additional_columns(Model & model);
 
 	template<typename FieldValue>
 	void put_field_value(const FieldValue & field_value, const FT & field_type)