Draft: Migrate to PySide6

After some struggles and investigation with the PyQt6 branch, I have decided to make the following changes.

1. Abandon attempts to make Wayland work - Use XWayland instead if on a wayland environment. For now at least, wayland does not provide the protocols we require for OpenLP to work (Window positioning, screen capture, and various other bugs, at least on a mutter desktop)
2. Rather than using PyQt6, move to Qt's official python bindings PySide6 which still allow the old enums (hopefully we get deprecation warnings if I missed any) and also has better documentation. (https://doc.qt.io/qtforpython-6/index.html). I should also note that it's under the same license as Qt (LGPL), I don't prefer any particular license but notably it is different than PyQt's GPL (or commercial).

The only significant difference changing to PySide6 was the runJavascript function does not provide a callback. I changed this to use the event system that was added somewhat recently.

Qt6 uses Wayland by default, so I am forcing the use of xcb on Wayland clients instead.

I've kept the enum changes, even though PySide6 supports the short forms, because I've read they may depreciate in the future.