QWebEngineCookieStorePrivate causes hard dependency on a exact QtCore version

QWebEngineCookieStorePrivate inherits QObjectPrivate, which causes a hard dependency of QtWebEngine on the exact QtCore version it was built with:

```

Fatal: Cannot mix incompatible Qt library (version 0x50903) with this library (version 0x50904)
KCrash: Application 'QupZilla' crashing...

```

That could be OK after a major/minor Qt upgrade, but seems a bit "too much" after a patch-level upgrade,. Especially for a probably non-crucial feature (cookie store) of a Qt component that provides functionality that can hardly be called "core" (no matter how important) and is possibly even more expensive to build than the full collection of pre-WebEngine Qt5 components. For me it's even more reason to avoid and dislike QWE, for other users it might be(come) a reason to hold off upgrading.

Wouldn't there be a more elegant way to implement QWebEngineCookieStorePrivate, making it inherit QObject for instance?