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/core/app.h

@@ -266,6 +266,10 @@ private:
 
 	void CreateStaticTree();
 
+	bool DoDatabaseMigration();
+	bool TryToMakeDatabaseMigration();
+
+
 	// !! IMPROVE ME
 	// !! move to the session manager?
 	time_t last_sessions_save;