load widgets only of enabled apps

load widgets only of enabled apps

- per design, all enabled apps have their registration run
- limitations, e.g. enabled by group, are not considered in that state,
  because we do not have a session (and might need apps?)
- before instantiation of widget it has to be checked whether the providing
  app is actually enabled for the logged in user.
- a public interface is being changed, but it is not meant to be
  implemented or used outside of the core handling. Therefore save to
  backport.

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>