feature: circle and square as polygon draw shapes

[jokd]

Fixes #2012
Configure the draw control like this to be able to draw squares and circles:

 {
  "name": "draw",
  "options": {
    "buttonText": "Rita",
    "drawTools": {
      "Polygon": ["freehand", "box", "square", "circle"],
      "LineString": ["freehand"]
    }
  }
}

[jokd]

The measurement values are perhaps a bit off since I use the circle radius and calculate it. Tested to convert the circle to a polygon and use the usual getArea function but the values seemed a bit low and I thought maybe it's because the circle getting chopped up in multiple segments.
Somebody with a bit of free time can do some control measuring and reply.

[asemoller]

Works fine!

I did some checking against a calculator and the area and perimeter are within the margin for rounding. So I guess it's ok or did you have something else in mind?

I'll submitt an issue about the problem with decimals in a day or two.

[huggla]

Works fine for me too.