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/dbexpression.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018-2021, Tomasz Sowa
+ * Copyright (c) 2018-2022, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -50,6 +50,10 @@ public:
 	DbExpression();
 	virtual ~DbExpression();
 
+
+	constexpr static const char * COLUMN_ROWS_COUNTER_POSTFIX = "_autoadded_rows_counter";
+
+
 	virtual void set_output_type(int output_type);
 	virtual int  get_output_type();
 
@@ -68,12 +72,13 @@ public:
 		std::wstring column_expression; // field() methods can be called recursively, so don't make it as class object
 
 		column_expression = new_column_expression;
-		column_expression += L" as ";
+		column_expression += L" AS ";
 		column_expression += new_column_name;
 
 		field(column_expression.c_str(), field_value, field_type, model_env);
 	}
 
+	virtual void generate_rows_counter_column_name(ModelEnv & model_env, pt::TextStream & str);
 
 
 protected:
@@ -97,12 +102,12 @@ protected:
 	void before_field_name();
 	void after_field_name();
 
-
-private:
+	void add_additional_columns(Model & model);
 
 	void before_field_value_string(const FT & field_type);
 	void after_field_value_string(const FT & field_type);
 
+	virtual void add_rows_counter_column(Model & model);
 
 };