Reimplement welcome page plugin using QWidgets

* QtQuick implementation bitrotted for many years:
  * only one area (Development) working
  * does not list sessions without any project
* a single UI library (QWidgets) across the app:
  * easier to maintain for developers, do not need to know 2 technologies
  * more consistent UI style
* less runtime/install resources needed
* follows other QWidget-using apps and their newer welcome pages also
  only using QWidets for that purpose