fix remaining issues for Qt6

[gdementen]

I did part of the work, but the more I test, the more issues I find. See #212.

Tentatively added to 0.35 milestone. Might be too early.

[gdementen]

I just tested using the latest qtconsole (5.4.4) and qtpy (2.4.0), installed using pip (the latest versions in anaconda are 5.4.2 and 2.2.0 respectively). This fixes an issue not mentioned in #212 (drag and dropping axis names to transpose) but does not fix the keyboard navigation issue I mentioned in #212. But, I could not find any other issue, so it might be worth fixing that keyboard issue.

I also tested updating (still via pip because qt6 is still not available on anaconda) pyqt6, pyqt6-qt6 and pyqt6-sip to their latest versions (6.5.2, 6.5.2 and 13.5.2 respectively) but did not find any difference.