Migration to Qt 6

[MangoTCF]

Most things look easily replaceable, except for GtkLayerShell. There is layer-shell-qt, but we would need to create the appropriate Python bindings for it to function with PySide. As far as a quick google goes, I couldn't find any other up-to-date implementations of the layer-shell protocol for Qt

[MangoTCF]

As per some more googling we can

• Reimplement the layer-shell using the existing PySide6 code, without any C++ involvement
• Create bindings for the C++ code in Python
  I don't really know which of these two options is preferable, but in any case this could be a separate package, on which SCC depends