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

samples/language.h

@@ -70,7 +70,7 @@ public:
 
 	void map_fields()
 	{
-		field(L"id", id, false, false, true);
+		field(L"id", id, FT::no_insertable | FT::no_updatable | FT::primary_key);
 		field(L"english_name", english_name);
 		field(L"local_name", local_name);
 		field(L"code_str", code_str);