Class to keep renderers around...

Each Cell in the grid, is a container UI component, and the actual renderer UI component The _dict object here holds a bucket for each container type/renderer type combination The _rendererFactories holds a map of each cell and the corresponding bucket key (FactoryKey) that was used to generate it, so we are able to put it back into the bucket that it came from.

The idea is, there should always be only just as many things in memory as are currently visibule plus a little outside so we scroll smoothly...