Deprecate (and remove?) CellStyle.backgroundView

[krbarnes]

The CellStyle.backgroundView property is too expensive. Views should only be created when they're needed. This background view property means that the view is created and destroyed on every render pass even though it's rarely used.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 3 days if no further activity occurs.