Add back KWallet support to Konqueror

Summary:
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.

I've not modified the copyright information in the files I copied from KDEWebKit as I don't know which is the correct way to do so: I must make clear my files are based on the work on other people but at the same time that I've changed them: how do I do this?

Test Plan: I tried to access several sites requiring a login in, asking Konqueror to save the password, then logging out and visiting the page again. Except when the autocomplete attribute was false (checked by looking at the html code), Konqueror always saved and restored login information correctly.

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D10178