Fix a bug causing being unable to overwrite existing cached data
This bug has been introduced by changing WebEngineWallet::WebEngineWalletPrivate::saveDataToCache so that it doesn't delete entries in pendingSaveRequests anymore but leaves this task to its caller. However, the caller can't know whether the data has actually been saved or whether the saveFormDataRequested signal has been emitted because the user needs to decide what to do. As a consequence, the entry in pendingSaveRequests will always be removed, so that when the user decides to save it, it won't be there anymore. To avoid this issue, saveDataToCache now returns a bool telling whether the data has been saved (and the entry must be remvoed) or not.