Add rvalue references of QSharedPointer cast operators

The various cast operations of QSharedPointer would benefit from move semantics: casting a rvalue QSharedPointer doesn't need to touch the reference counter. Right now instead a copy is always taken.

This is just FYI, I know QSharedPointer isn't getting love, and likely won't happen before QTBUG-109570 (i.e. QSharedPointer reimplemented as a wrapper around std::shared_ptr, that is, Qt 7). People should stop using QSharedPointer already, except that we expose it in APIs here and there (mostly as API flaws).