Provide a shorthand for using a deleteLater-based deleter in std::{shared,unique}_ptr

Like QScopedPointerDeleteLater for QScopedPointer, but with a name that has no connection to QScopedPointer. e.g. QObject::DeleteLaterDeleter as suggested by Marc Mutz.

While I'm not suggesting we delete QScopedPointer any time soon, if we would ever want to do it then we should have some documented way to replace it with unique_ptr that doesn't still include using ScopedPointer facilities.