added support for Model migrations

now we have a table core.migration and each model (User, Group, Item, ItemContent and a new Migration) have its own
row in the table with a version number

added to config:
db_make_migration_if_needed and db_stop_if_migration_fails (need description yet)

winixd/models/group.h

@@ -82,6 +82,16 @@ public:
 		name.clear();
 		//members.clear();
 	}
+
+
+	bool do_migration(int & current_table_version);
+
+
+protected:
+
+	bool do_migration_to_1();
+	bool do_migration_to_2();
+
 	
 };