allow to custom title_item thanks to config

As it is said in the comment, the title_item() method allow to define a callback to define title_property in a way that this callback would retrieve a value instead of a property.
In my opinion, this is a mistake. Let's consider something like building a query and doing something like ->order_by($class::title_property()) : could be great but does not work with a callback. Even if the callback is executed before (would do orber_by(value) instead of order_by(property)).

I kept this piece of code as I do not know if it's used and do not want to introduce a breaking change. Should be done later though.