Remove pointless QPointer and QWeakPointer from public api

QPointer is not cheap if it's constructed a lot, exposing QPointer in
public API is a red flag in general.

In addition to that, the associated client object cannot be just yanked,
Decoration::client() always returns a non null value, so QWeakPointer
adds no real value. The code that uses Decoration::client() calls
QWeakPointer::toStrongRef() and performs no null checks.