[For 0.19] support QWebEngine and QWebKit refs
Created by: mumme74
This pull request implements building against QWebEngine if Qt is >= 5.7 I fallbacks to QWebkit if qt is <= 5.6
You can configure to force use QWebKit with: cmake ... -DFREECAD_USE_QWEBKIT=ON ...
This was discussed here https://forum.freecadweb.org/viewtopic.php?f=10&t=26834#p279083
I have tested it on linux Qt4.8, Qt5.12 WebEngine and Webkit I havent tested the QWebkit part on windows or macOS, just the QWebEngine part. (I have not yet succeded building QWebKit with Msvc2017..)
References: issue #3774, issue #2986