Makes hover detail arrow rendering more reliable

Uses the 'CSS triangle' trick to render the left and right arrows on the
hover detail item, rather than using a glyph (whose rendering might
change depending on the font used).

The technique is described here:

http://appendto.com/blog/2013/03/pure-css-triangles-explained/
Merge pull request #334 from kungfubanana/hover-detail-arrow-rendering

Makes hover detail arrow rendering more reliable