misc: remove dead code and add support for Qt 6

CMakeLists.txt

po/POTFILES.in

src/accountcreationwizard.cpp

src/mainwindow.cpp

src/models/activeitemproxymodel.cpp

src/models/activeitemproxymodel.h

src/models/gtkaccessproxymodel.cpp

src/models/gtkaccessproxymodel.h

src/models/gtkqtreemodel.cpp

src/models/gtkqtreemodel.h

src/usernameregistrationbox.cpp

src/utils/models.cpp

src/utils/models.h

src/video/video_widget.cpp