Specs: update the layout and paint schemes to match discussions better

sky/examples/style/toolbar-layout.sky

 SKY MODULE
 <import src="sky:core" as="sky"/>
 <script>
  // display: toolbar;
  // toolbar-spacing: <length>
  // display: spring; // remaining space is split equally amongst the springs
  // children are vertically centered, layout out left-to-right with toolbar-spacing space between them
  // last child is hidden by default unless there's not enough room for the others, then it's shown last, right-aligned
  module.exports.SpringLayoutManager = class SpringLayoutManager extends sky.LayoutManager { }
  sky.registerLayoutManager('spring', module.exports.SpringLayoutManager);
@@ skipping 34 matching lines @@
      let pendingSpacing = 0;
      children = this.walkChildren();
      loop = children.next();
      while (!loop.done) {
        let child = loop.value;
        let dims = null;
        if (child.layoutManager instanceof module.exports.SpringLayoutManager) {
          springCount += 1;
          pendingSpacing = spacing; // not +=, because we only have one extra spacing per batch of springs
        } else {
-         if (child.needsLayout) {
+         if (child.needsLayout || child.descendantNeedsLayout) {
            childHeight = child.layoutManager.getIntrinsicHeight();
            if (childHeight.value < height)
              childHeight = childHeight.value;
            else
              childHeight = height;
            dims = child.layoutManager.layout(width, height);
            this.setChildSize(child, dims.width, dims.height);
          } else {
            dims = {
              width: child.width,
@@ skipping 112 matching lines @@
      if (result.minimum > result.maximum)
        result.maximum = result.minimum;
      if (result.value > result.maximum)
        result.value = result.maximum;
      if (result.value < result.minimum)
        result.value = result.minimum;
      return result;
    }
    function paintChildren(canvas) {
      let width = this.node.width;
+     let children = this.walkChildren();
      let loop = children.next();
      while ((!loop.done) && (loop.value != this.firstSkippedChild))
        this.paintChild(loop.value, canvas);
      if (this.showingOverflow)
        this.paintChild(this.overflowChild, canvas);
    }
-   function paintChild(child, canvas) {
-     if (child.needsPaint) {
-       canvas.save();
-       try {
-         canvas.beginPath();
-         canvas.translate(child.x, child.y);
-         canvas.rect(0, 0, child.width, child.height);
-         canvas.clip();
-         child.paint(canvas);
-       } finally {
-         canvas.restore();
-       }
-     }
+   function inChild(child, x, y) {
+     return (x >= child.x) && (y >= child.y) && (x < child.x+child.width) && (y < child.y+child.height);
+   }
+   function hitTest(x, y) {
+     let children = this.walkChildrenBackwards();
+     let loop = children.next();
+     while ((!loop.done) && (loop.value != this.firstSkippedChild))
+       if (this.inChild(loop.value, x, y))
+         return loop.value;
+     if (this.showingOverflow)
+       if (this.inChild(this.overflowChild, x, y))
+         return this.overflowChild;
    }
  }
  sky.registerLayoutManager('toolbar', module.exports.ToolbarLayoutManager);
 </script>