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.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018, Tomasz Sowa
+ * Copyright (c) 2018-2021, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -86,7 +86,7 @@ protected:
 	std::vector<int> conjunctions;
 
 
-	bool can_field_be_generated(bool insertable, bool updatable, bool is_primary_key);
+	bool can_field_be_generated(FT field_type);
 
 	void field_before();