Revert "Using event registration for IndexedDB API callbacks"

This reverts commit c70faaf0a4003c0dd620e0b9b50347cfb6b17571.
Reimplemented event registration for IndexedDB API callbacks

The IndexedDB API specifies two methods for event registration on its
interfaces. Direct callback funtions and the EventTarget scheme.

To make the interface as standards conformant as possible, I derive
classes providing callbacks from IDBEventTarget base class (proviedes
add-/removeEventListener) and provide fields for direct callback
functions.