Switch imports to `qtpy`

[trappitsch]

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

• [x] 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