Add support for autoloading of *_Ui_VarRenderer_* classes

[amulet1]

To utilize it we just need to rename Horde_Core_Ui_VarRenderer_* classes in other apps (except Core).

For example, in Turba app class Horde_Core_Ui_VarRenderer_Turba should be renamed to Turba_Core_Ui_VarRenderer_Turba.

To provide a seamless transition a fallback to the original loading method is implemented. Once it is done, it can be removed:

// TODO: Eliminate after renaming Horde_Ui_VarRenderer_* classes in other apps to {app}_Ui_VarRenderer_*
if (!$ok && !empty($app)) {
    // fallback to legacy method (manual load)
    $class = __CLASS__ . '_' . $driver;
    include_once $GLOBALS['registry']->get('fileroot', $app) . '/lib/Ui/VarRenderer/' . $driver . '.php';
    $ok = class_exists($class);
}

[amulet1]

@ralflang, please merge.

[amulet1]

@TDannhauer, please could you review/merge?

[TDannhauer]

AI supported, human reviewed and tested in deployment. looks good for me. Adaptions in apps are handled separately, fallback allows transition.