When grouping for '% done' in the WorkPackage table the progress bar has issues

* when hovering over the progress bar in the group-colum, it gets highlighted in yellow (which looks silly)
* the progress is not displayed as text

See: https://www.openproject.org/work_packages/15608

|old|new|
|---|----|
| ![pmxmxexqtr](https://cloud.githubusercontent.com/assets/206108/4475966/87106b70-4971-11e4-94c9-5b56a6f08ae1.png) |  ![selection_055](https://cloud.githubusercontent.com/assets/206108/4475942/59c39f02-4971-11e4-9375-97460a2c7525.png) |