Add designer support for custom widgets when running on pyside6

[nstelter-slac]

What's the problem this feature will solve?

Need to add full support for loading widgets into pyside6-designer when running on pyside6.

Describe the solution you'd like.

references for doing this:

https://doc.qt.io/qtforpython-6/tools/pyside-designer.html#custom-widgets
https://doc.qt.io/qtforpython-6/examples/example_designer_taskmenuextension.html#example-designer-taskmenuextension

Additional context.

No response

[nstelter-slac]

picking this up, i've got this part-way implemented already in a local branch