text: Title: Add suport for touch event callbacks
  Body: Hiya,
  
  Love the library - interactive notebooks are the future! 
  
  I managed to whip up a really simple drawing-pad program on my laptop that worked well in Jupyter Lab, however when I tried loading the same notebook on a tablet (tried Firefox and Chrome) whilst it seems like `mouse_down` events are triggered, dragging didn't work, so drawing didn't work. I wonder whether adding the set of Touch Events might solve this problem: 
  https://developer.mozilla.org/en-US/docs/Web/Events#Touch_events
  
  Thanks,
  
  Hugh
  
  