Finish Horde_Prefs rewrite

Finish Horde_Prefs rewrite

Main focus: ability to to stack storage drivers to allow for multiple
drivers to build a preferences scope.

Horde now uses 3 storage drivers to build the final prefs hash: the
configuration file, prefs backend, and hooks.

Caching needed to be separated from storage since the two objects do
*not* store the same data.  Caching is intended to store the entire
preference scope, storage only handles discrete preference values.

Default/dirty bits are now entirely handled internally by the Scope
object.

Remove all external charset conversion API for prefs.  Charset
conversion, if needed, should be done entirely within the storage
driver (prefs now assumes all in-memory prefs are in UTF-8).