Modifying GUI

 **Please do not hand edit .ui files, as qt's xml formt is fragile and its
 tools are inconsistent across platforms on tolerance to bad constructs.**

 Instead prefer qdesigner5 to make edits to the .ui documents.  If you are 
 allergic to gui tools, then **at least**, after making hand edits, prior to
 checking them in, load the file in qdesigner5, ensure it loads without 
 error, and then **save the file back out, and check that in**, so that the
 next developer using qdesigner5 does not contend with unrelated diffs.

Testing

Please run the tests in pxr/usdImaging/bin/testusdview, and add new ones whenever possible.
See blackBoxTesting.md, and please add to it when adding features that cannot be tested by testusdview