Mac: Initial code to transition from NSCell based container/tree controls. Been getting deprecation warnings about this since Yosemite 10.10 but the code has been functional so I had been ignoring it. In Big Sur 11.0 the cell based version is displaying incorrectly, instead of spending effort to fix the deprecated cell based version, investing time switching to the View based version using NSTableCellView. This code semi-works, displays briefly before getting an invalid instruction in the message loop. Committing as BUILDING_FOR_YOSEMITE1 instead of BUILDING_FOR_YOSEMITE so the code is currently disabled.