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.cpp

@@ -126,6 +126,7 @@ void BaseExpression::generate_from_model(Model & model)
 		before_generate_from_model();
 		dump_additional_info(model);
 		model.fields();
+		add_additional_columns(model);
 		after_generate_from_model();
 	}
 }
@@ -141,6 +142,11 @@ void BaseExpression::dump_additional_info(Model & model)
 }
 
 
+void BaseExpression::add_additional_columns(Model & model)
+{
+}
+
+
 void BaseExpression::before_generate_from_model()
 {
 	is_first_field = true;