plugins onAutoload now only overloads if necessary (extlibs etc.)

plugins onAutoload now only overloads if necessary (extlibs etc.)

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).