Specs: Define when needsLayout is reset (also, some unrelated markdown fixes)

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);
  sky.registerProperty({
    name: 'toolbar-spacing',
    type: sky.PositiveLengthStyleValueType,
    inherits: true,
    initialValue: 8,
    needsLayout: true,
  });
  module.exports.ToolbarLayoutManager = class ToolbarLayoutManager extends sky.LayoutManager {
    constructor (styleNode) {
      super(styleNode);
      this.showingOverflow = false;
      this.firstSkippedChild = null;
      this.overflowChild = null;
    }
    function layout(width, height) {
+     this.markAsLaidOut();
      let children = null;
      let loop = null;
      if (height == null)
        height = this.getIntrinsicHeight().value;
      if (width == null)
        this.assumeDimensions(0, height);
      else
        this.assumeDimensions(width, height);
      let spacing = this.node.getProperty('toolbar-spacing');
      if (typeof spacing != 'number')
@@ skipping 164 matching lines @@
          canvas.clip();
          child.paint(canvas);
        } finally {
          canvas.restore();
        }
      }
    }
  }
  sky.registerLayoutManager('toolbar', module.exports.ToolbarLayoutManager);
 </script>