Added back the ability to store user names and passwords to KWallet and automatically fill them when loading a page.
I copied the files `kwebwallet.cpp` and `kwebwallet.h` from the `KDEWebKit` framework and modified it to adapt to `QWebEngine`. In particular, I needed to modify the way form data is passed from javascript to C++ serializing it using JSON and read it back using `QJsonDocument`. Also, since `QWebEnginePage` doesn't allow to access frames from the C++ side, I needed to remove all the code dealing with recurisve searches and implement it in javascript.
As the old code, this intentionally doesn't work with input elements with the `autocomplete` attribute set to `false`.