#858 item zoomfactor should be calculated from its group

[High Performance Coder]

An item's zoomfactor is basically the product of its group zoomfactor and a local group scale factor that will also determine the displayZoom value. This can be calulated at resize time, or possibly be made manually adjustable.

Ultimately, the zoomFactor attribute of Item should disappear, being replaced by recursively multiplying the local group zoom factors as you walk up the group heirarchy.