title: Highcharts: replace custom button image on hover
body: <p>I added a new custom button, and wanted to change it's fill when mouse hover.
  Can it be done when using symbol image? 
  If not I have another symbol that looks the same but has different fill color
  so it will look like hovering? is there a way to replace the image upon mouseenter/mouseout?</p>
  
  <p>Regards
  Chanan</p>