add clickToHighlight to gCNV track

[jethror1]

Adds clickToHighlight option to gCNV track for setting sample line colour on click.

Accepts either a single fixed colour or a keyword any that sets a random colour for each line.

Line colour is unset on clicking the same line again.

Closes #1411

Example of clickToHighlight=red:

Example of clickToHighlight=any:

[jrobinso]

This is a rather extreme method to force a repaint

igv.visibilityChange()  // prompt redraw to change colour of clicked sample

Try this instead

this.trackView.repaintViews();

Where "this" is the track object itself

[jethror1]

Have changed it, I wasn't sure on the best way to prompt redraw and used your suggestion from here: #756 (comment)

[jrobinso]

Yes I should document this, apologies for that. resize() and visibilityChange() will work but also potentially result in repainting all tracks, not just the one affected, and also potentially a data reload, not just a repaint.