Improved database engine plugins loading

.db.php files directly on the plugins path will now load.
Added placeholder for extra config. path

The configuration field on the category paladio-path has been supported
for a long time, yet it was not being mentioned.
AccessControl will look for 'role:*' categories

AccessControl used to look for categories on the form 'rol:*' instead of
'role:*'. This has changed.
Improved support for AccessControl config path

The configuration at the given path will only be loaded once instead of
trying to load it after each configuration load.

Note: this is used to load .nav.php files (these files form the acl),
you can have as many as you want.