[BUGFIX] Changed query to use criteria instead of source

Doctrine-dbal implementation of PHPCR does not allow to select FROM a
mixin type. This fix moves the node type specification to the criteria.

Note that this should be reverted when doctrine-dbal does support it, as
it can be more performant (but current benchmarks do not indicate a
performance penalty on jackrabbit).
Until now we have used `TODO` instead of `@todo`, would that be also ok for? Otherwise we are quite inconsistent.