Implement a unified preload file in some modules

Allow all preload items to be specified in a single file. Previously there was a 'core.php' file for 'core' events, a 'system.php' for 'system' events, etc. This change allows one file, name 'preload.php' to contain all the events a module is interested in. Accordingly, the class name in this file no longer contains the event source. Event and function names are unchanged.

For more information, see htdocs/xoops_lib/Xoops/Core/Events.php
Allow absolute urls in button links
Add RotatingFileHandler option for logs versioned by date
Doctrine conversion missed earlier
Declare classes as abstract

Affects XoopsObject,  XoopsObjectHandler and XoopsObjectPersistableHandler