Add Horde_Auth_Application callback feature.

Add Horde_Auth_Application callback feature.

Using application auth, especially if using application auth as horde
auth, there was previously a severe limitation in that the application
only had a limited environment when doing the auth.  This makes sense
because, since no user was yet logged in, there was no way to init
things such as prefs.  Best (only?) solution is to understand that
application auth only has limited environment and should be used to do
the minimum necessary to verify authentication only.  Login tasks and
the like need a full environment, so add a authentication callback hook
that will re-call the application after a full environment can be setup.