add fileset file addition and removal events

added two events:
on_file_added_to_set($file_id, $file_set_id)
on_file_removed_from_set($file_id, $file_set_id)

it seems when using the UI and updating the sets a file is in
all associations are removed and then re-added based on the checkboxes
which causes the on_file_added_to_set event to be triggered multiple
times for sets the file is already in. However, when called
programatically these events will be fired as expected