Fix coverity metrics.

Count only outstanding issues: issues which are neither fixed nor dismissed.
Store name in XML file to improve sonar startup time.

This reduces coverity rules load time from ~20 seconds to less than 1 second.
Rewrite coverity widget & metrics.

- Metrics are now integers and qualitive, to allow tracking evolution.
- Widget gets a look similar to other widgets, with outstanding issues shown
prominently and distribution by impact on the side.
- Support variation display.
- Cleanup management of links, by storing them in url field of the measure.
- A piechart is also present to graphically show the distribution.