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

sky/examples/style/block-layout.sky

 SKY MODULE
 <import src="sky:core" as="sky"/>
 <!--
  ! this module provides trivial vertical block layout
  ! no margins, padding, borders, etc
  !-->
 <script>
  module.exports.BlockLayoutManager = class BlockLayoutManager extends sky.LayoutManager {
    function layout(width, height) {
      this.markAsLaidOut();
      if (width == null)
        width = this.getIntrinsicWidth().value;
      let autoHeight = false;
      if (height == null) {
        height = 0;
        autoHeight = true;
      }
      this.assumeDimensions(width, height);
      let children = this.walkChildren();
      let loop = children.next();
      let y = 0;
      while (!loop.done) {
        let child = loop.value;
-       if (child.needsLayout) {
+       if (child.needsLayout || child.descendantNeedsLayout) {
          let dims = child.layoutManager.layout(width, null);
          this.setChildSize(child, dims.width, dims.height);
        }
        this.setChildPosition(child, 0, y);
        y += child.height;
        loop = children.next();
      }
      if (autoHeight)
        height = y;
      return {
@@ skipping 31 matching lines @@
          if (height < childHeight.value)
            height = childHeight.value;
          loop = children.next();
        }
      }
      return super(height); // applies and provides our own min-height/max-height rules
    }
  }
  sky.registerLayoutManager('block', module.exports.BlockLayoutManager);
 </script>