added FT class which is used in Model::field() methods

FT class has following types:
   enum FieldType
   {
	default_type = 0,
        primary_key = 1,
	foreign_key = 2,
	foreign_key_in_child = 4,
	no_insertable = 8,
	no_updatable = 16,
	no_fetchable = 32, /* not supported yet */
   };
an object of FT class are now used in Model::field() methods instead of insertable/updatable/is_primary_key/... boolean flags

changed the semantic of has_foreign_key (which was a bool) flag in child Models:
now on Models and list/vector of Models you should use either FT::foreign_key or FT::foreign_key_in_child
1. FT::foreign_key means that field with this flag is a foreign key and is pointing to the child object
   (it was the case when has_foreign_key was equal to true beforehand)
2. FT::foreign_key_in child means that the foreign key is in the child object and is pointing to the parent object

src/dbexpression.cpp

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018-2019, Tomasz Sowa
+ * Copyright (c) 2018-2021, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -62,23 +62,23 @@ int DbExpression::get_output_type()
 }
 
 
-bool DbExpression::can_field_be_generated(bool insertable, bool updatable, bool is_primary_key)
+bool DbExpression::can_field_be_generated(FT field_type)
 {
 	if( output_type == MORM_OUTPUT_TYPE_DB_INSERT )
 	{
-		return insertable;
+		return field_type.is_insertable();
 	}
 	else
 	if(	output_type == MORM_OUTPUT_TYPE_DB_UPDATE )
 	{
-		return updatable;
+		return field_type.is_updatable();
 	}
 	else
 	if( output_type == MORM_OUTPUT_TYPE_DB_PRIMARY_KEY ||
 		output_type == MORM_OUTPUT_TYPE_JOIN_TABLES ||
 		output_type == MORM_OUTPUT_TYPE_DB_INSERT_PRIMARY_KEY )
 	{
-		return is_primary_key;
+		return field_type.is_primary_key();
 	}
 	else
 	{