GUI: Use qtpy compat layer rather than PySide6 in imports #3356

Is your feature request related to a problem? Please describe.

For compatibility and ease of use, using qtpy rather than importing from PySide6 is considered better. Note that this does not prevent us at all from specifying PySide6 as a dep or using it in tests.
Doing this around the time we introduce proper GUI tests would likely be worth it.

Describe the solution you'd like

Replace all PySide6 calls with qtpy. The same should be done in the napari plugin.