Fix for #980.

The code in thunar_text_renderer that tackled the issue of overlapping highlights (one is the normal highlight and the other is the custom highlight) on selected file names could/should have caused the issue with #980 (closed).

This MR should fix #980 (closed) and is also a better hack to solve the overlapping highlights. The earlier hack used cairo to hide the normal highlight of the selected file names whereas with the new hack we are using css to set the border-radius.