WIP: Port to PySide6

[jsvgoncalves]

[!NOTE]
There are two commits for easier review, I'll squash them once the PR is approved.

Port to PySide6

• Update imports to reference PySide6
• Update QAction imports, it has moved from QtWidgets to QtGui
• Signal handling in PyQt6 is more generic and the activated signal emits only the index, see: https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/QComboBox.html#PySide6.QtWidgets.PySide6.QtWidgets.QComboBox.activated

For general porting notes, see: https://doc.qt.io/qtforpython-6/gettingstarted/porting_from2.html