Using new changingHierarchy attribute to mark children that appear/disappear over time.

The previous behaviour used scene:visible exclusively for this test, which ran into issues when trying to pass that attribute through from the liveScene.
Making SceneInterface::visibilityName a reference
Using SceneInterface::visibilityName now that it exists
Merge pull request #280 from andrewkaufman/visibileCrash

Fixing crash in SceneCache ROP todo with "scene:visible" attribute