Show a proper mouse pointer when hovering axis labels.
Allow for scale inversion on double click.

When it comes to seeing correlations, research has shown that negative
correlation are easier to see in parallel coordinates than possitve
(http://www.cs.tufts.edu/~remco/publications/2014/InfoVis2014-JND.pdf).
Therefore, I added the ability to easily inverse the scale of an axis.
Make sure that shadows are re-rendered on axes reordering.

I'm not a big fan of the shadows functionality, especially because it's
current implementation is not suitable for a large number of items.
However, if it's used, it should be at least working correctly. Before this
fix, the shadows wouldn't update on axis reordering, which gave a strange
(and incorrect) effect.