Store foreign keys, even not supported

At the moment foreign key constraints are only stored, if the table engine supports foreign keys.
E.g. for Aria or MyISAM you can define them, they are not respected nor stored.

If at least the information is stored, applications could use this information to visualize dependend tables or to manually check some constraints by querying the "Information Schema" before/after delete.

This could help making the database more consistent while Aria does not support transaction (and maybe never will). An offline job could also be used to check for consistency breaks.