Add template typing to the QBMapper

The QBMapper is kind of a generic type, though this concept does not
exist in php. Hence you have a lot of type coercion in subtypes (mappers
in the individual apps) because you suddenly don't expect an Entity[]
but your specific type.

Luckily Psalm lets us type those. Then in the subclass you can
psalm-implement the mapper with a concrete type and psalm will do all
the magic to ensure types are used correctly.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>