Core API: "prevent default" for Core events

[jri]

A plugin listener, e.g. preUpdateAssociation(), should have the ability to fully take over control, and prevent the Core's default behavior, that in this case is storing the updated association in the DB.

Without a "prevent default" mechanism the Core API and implementation becomes quite ugly when it comes to the association's double nature. This becomes notorious in case of editing association definitions manually. At one hand an association definition has the behavior of a generic association, at the other hand it has special obligations regarding the cached type definitions.

[jri]

Meanwhile we found other means to deal with the "association's double nature".