[widePlotter] Fixed behavior of the highlighter missing points in averaging mode.

Now it always highlights the next closest point to the mouse's location,
whereas before it would only highlight a point if the mouse was directly
above it. This would cause problems in averaging mode where there are
typically <1 points per pixel.
[widePlotter] The highlighter now has an option to highlight phase.

You can toggle the option to highlight phase instead of magnitude
by pressing the 'h' key. The default is set to highlight magnitude.
This of course only applies to the cross-correlations, the autos
will always highlight amplitude.