Switch imports to `qtpy`

[trappitsch]

For simpler development, imports should move from PyQt6 to qtpy. A small todo list to achieve this:

• Check if it works with fbs
• Switch all imports
• Test if it runs with PySide
• Test if it runs with PyQt5
• Update docs
• Test compiled PySide package on Windows

[trappitsch]

fbs compatibility has been successfully shown in other projects, e.g., DigOutBox