Feature: Make control char serve as aging chart

[stepev]

For managing flow it's crucial to control work item aging. One way is to visualize aging directly on the card. But it also would be useful to have an overview.

The solution could be, on the control chart:
Use color backgrounds to highlight wip age percentile zones.

Percentiles could be calculated automatically based on the completed task through the period, accounting quick filters.

Example

Additional features and tunings:

• allow to define health zones by specifying percentile thresholds
• Feature: for the control chart add built in filters wip/done #210
• Feature: visually designate WIP/done dots on the control chart #209