The Event plugin is still using a deprecated hook for saving extra data,
which makes it harder to hook into from other plugins. This commit
fixes it to use the new hook for at least RSVPs.
NOTE: this plugin incorrectly sets the object_type of RSVP notices to
their verb. This was existing behaviour which I have preserved for
backwards-compatability.
lib/plugin.php now has a parent onAutoload function that finds most common
files that are used in plugins (actions, dataobjects, forms, libs etc.) if
they are put in the standardised directories ('actions', 'classes', 'forms',
'lib' and perhaps some others in the future).
