Add better warning or guard when running plugin UI actions off the main thread

[fuzyll]

See #6804 for a good example of how customers can get tripped up by this, although it happens pretty often. We've got this documentation, but that doesn't prevent users (or developers lol) from accidentally making mistakes.

Not really sure how to do this, but one thing that was floated at a recent meeting that I liked was modifying shiboken and checking the thread ID before/as an API call is made.