As of master, the qt editor is broken when editing an axes that contains
an image throws an error when clicking OK. This is becauseself.data
needs to be modifiedin-place in FormWidget.setup, as FormWidget.get
later checks some explicit types in there.

This means, as a side note, that passing a tuple instead of a list to
generate a QComboBox is broken (one can test that by modifying the
example at the bottom of the module -- the results are different
depending on whether 'list2' is a list or a tuple) but that's something
we can deal with some other day...

Partial revert of#11684 (well, I naively assumed that the base code was
correct...).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way