playground structure for basic doodeling

.swift

//at the very top of the class
import Foundation
import UIKit
import PlaygroundSupport

public class MyDoodleCanvas {
    ...
}

//after the class implementation
var canvasView = UIView(frame: CGRect(x: 0, y: 0, width: 450, height: 630))

var myCanvas = MyDoodleCanvas()
myCanvas.backgroundColor = .white
myCanvas.isUserInteractionEnabled = true
myCanvas.frame = canvasView.frame

canvasView.addSubview(myCanvas)

//most important: tell the playground what to display!
PlaygroundPage.current.liveView = canvasView