Add support for tooltips schema property

[SamGRosen]

Tooltips are an important aspect for reading visualization. Most of the functionality is already present with WebGLVis.getClosestPoint. This method outputs to WebGLVis.dataWorkerStream and can be used for displaying the tooltip.

Implement the tooltips schema property on a track level where a value of 0 is no tooltips, 1 is always show tooltips, and any number between is begin showing tooltips when zoomed to that level, e.g. a value of 0.5 means show tooltips when zoomed in to half of the domains.

Example schema:

{
  "defaultData": {
    "day": [1, 2, 3],  
    "price": [10, 22, 35],
  },
 "tracks": [
  {
    "tooltips": 1, // always show tooltips
    "tooltips": 0, // never show tooltips
    "tooltips": 0.1, // show when zoomed in to 10% of domains.
    "mark": "line",
    "x": {
      "attribute": "day",
      "type": "quantitative",
      "domain": [1, 10]
    },
    "y": {
      "attribute": "price",
      "type": "quantitative",
      "domain": [0, 40],
    },
  }
 ]
}