…on when single-picking (#143)

* HYDRA-1058 : Rough basic structure for GeomSubset picking

* HYDRA-1058 : Attempt to add menu item to USD selection mode menu

* HYDRA-1058 : Functioning registering of menu items in USD selection menu

* HYDRA-1058 : Functional picking mode menu

* HYDRA-1058 : Cleanup picking code

* HYDRA-1058 : Cleanup pick interpretation code

* HYDRA-1058 : Move comment

* HYDRA-1058 : Avoid unnecessary idle events to create pick mode UI

* HYDRA-1058 : Move all GeomSubset MEL code into same script

* HYDRA-1058 : Cleanup GeomSubsets UI setup code

* HYDRA-1058 : Tear down UI on plugin unload and if MayaUSD is unloaded, and reconstruct it if MayaUSD is reloaded

* HYDRA-1058 : Simplify data keeping for UI teardown

* HYDRA-1058 : Remove print debugging statements

* HYDRA-1058 : Reorder operations

* HYDRA-1058 : Cleanup headers and rename method

* HYDRA-1058 : Add comments to explain how the pick mode menu is hooked up

* HYDRA-1058 : Remove headers

* HYDRA-1058 : Rename prims picking mode

* HYDRA-1058 : Adjust UI label

* HYDRA-1058 : Clarify naming

* HYDRA-1058 : Add comments

* HYDRA-1058 : Handle single selection pick mode

* HYDRA-1058 : Fix UI deletion and re-creation

* HYDRA-1058 : Remove select menu callback on UI teardown

* HYDRA-1058 : Fix callbacks clear command

* HYDRA-1058 : Fix UI deletion on plugin unload and re-load

* HYDRA-1058 : Remove prints

* HYDRA-1058 : Extend check for UI creation for consistency

* HYDRA-1058 : Add example scene

* HYDRA-1058 : Fix GeomSubsets pick resolution for Xform parent

* HYDRA-1058 : Use anonymous namespace variables instead of static

* HYDRA-1058 : Refactor plugin loading callbacks registration

* HYDRA-1058 : Remove superfluous UI code changes

* HYDRA-1058 : Add upper half geom subset sphere scene

* HYDRA-1058 : Simplify GeomSubset prim

* HYDRA-1058 : Add upper half geom subset cube scene

* HYDRA-1058 : Simplify cube scene

* HYDRA-1058 : Fixup

* HYDRA-1058 : Scene with instancing and materials

* HYDRA-1058 : Adjust scene

* HYDRA-1058 : Add test files

* HYDRA-1058 : Adjust test scene

* HYDRA-1058 : GeomSubsets picking marquee select test

* HYDRA-1058 : GeomSubset picking test

* HYDRA-1058 : Fallback picking test

* HYDRA-1058 : Instance GeomSubset picking test

* HYDRA-1058 : Instance fallback picking test

* HYDRA-1058 : Refactor and cleanup testUtils

* HYDRA-1058 : Cleanup testGeomSubsetsPicking.py

* HYDRA-1058 : Remove dead code

* HYDRA-1058 : Fixup

* HYDRA-1058 : Extract out common test code

* HYDRA-1058 : Extract out common test code for instance tests

* HYDRA-1058 : Cleanup tests

* HYDRA-1058 : Cleanup test headers and increase thoroughness

* HYDRA-1058 : Remove unused file

* HYDRA-1058 : Rename picking modes and use token-based approach for extensibility

* HYDRA-1058 : Small touchups

* HYDRA-1058 : Cleanup GeomSubsets mel file

* HYDRA-1058 : Add localization support

* HYDRA-1058 : Add copyright notices

* HYDRA-1058 : Add comments in test

* HYDRA-1058 : Handle build support for USD < 2403

* HYDRA-1058 : Skip GeomSubset picking tests if USD version does not support GeomSubset prims

* HYDRA-1058 : Debug OSX and Linux failure #1

* HYDRA-1058 : Debug OSX and Linux #2

* MAYA-xxxx <one line description>

* HYDRA-1058 : Debug OSX #1

* HYDRA-1058 : Adjust test setup to use markers for clicking

* HYDRA-1058 : Rename resolvePrimAndInstancePicking

* HYDRA-1058 : Fix typo

* HYDRA-1058 : Revert obsolete changes

* HYDRA-1058 : Cleanup testGeomSubsetsPicking.cpp

* HYDRA-1058 : Add link to MayaUSD register strings MEL file

* HYDRA-1058 : Add comment