Hotspot "type" on sketches

I like this implementation pretty well.  The hotspot type adds a class that makes all sketches transparent until highlighted, at which point they are 40% opaque.  Any sketch object can be a hotspot, but you have to fill them if you want them to be clickable over their whole area.  The fill implementation is by shape (group) but should probably be item by item. fill: 'White' is best for hotspots I think.