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.

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