USD Import: Visible Prims Only option

[kowalskima]

It would be useful to prune invisible prims from the traversal. I've added a Visible Prims Only import option to enable this feature. This flag only applies to prims with a non-animating visibility attribute. I.e., prims with animating visibility will always be imported.

I've pushed this change to the usd-importer-T81257-merge branch and have attached a visibilityTest.usda file for testing below.

visibilityTest.zip