refactored pyqt6 to Pyside6

[JTvD]

Implements #293. There have been a few attempts to move to pyside before, unfortunately there are a few tricky changes between pyqt and pyside.
One of them is the custom UiLoader needed to load UI files in pyside6 like done in pyqt. I have not found any function in the pyside6 library that could be used as alternative.
A second challenge is that the QPlainTextEditLogger, which inherits the 'PySide6.QtCore.QObject' and must therefore be running on the main thread was emitting signals to the qtextedit. This is not needed as a class that has a reference to a widget must be on the mainthread.

Regardless, I changed it is because pyside6 signals do not directly slot into the QT slot like the signals of pyqt6.
there are quite a few topics about this on stackoverflow . For as far as I saw this is the only place where it was done, if it is needed else where a simple pyside6 slot that passes the string to the widget suffices.

Based on a few hours of testing it seems to be working. Nevertheless it is recommended to test the code thoroughly.

Note, the code to create the new ui_files follows in a second pull request.

[qubixes]

@JTvD Thanks a lot! I can confirm that everything seems to work that I have tried so far (which is not much).