The four blob operation structures defined in blob.c are created by
a macro which initializes the function pointer for the event handler
to ${name}_event_handler, where $name is the blob type, i.e. one of
images, lyrics, moods, playlists.
Only two of the four, moods and playlists, need an event handler
because the images and lyrics tables ignore events. Currently we
have to define dummy functions {images,lyrics}_event_handler() to
avoid link errors. This extra code can easily be avoided by making
the macros a little smarter.