fix wp lines jump when hover over grouping

[``* `#16817` wp lines jump when hover over grouping``](http://community.openproject.org/work_packages/16817)

I had the same on chrome on mac. I think the problem is related to rounding of height depending on the font-size. This can be fixed either by setting the font-size of the link to 74% or 90% or by changing the display: none/inline to visibility: hidden/visible (this way the element will be invisible but will occupy the same space on the page rendering the same size of the TD