BlLayoutNode hierarchy: convert traits to classes

[tinchodias]

In the following hierarchy:

this happens:

• The superclass is EMPTY instance and class side
• Half of the subclasses have traits: {TBlLayoutNode + TBlLayoutChildNode}
• The other half have: traits: {TBlLayoutNode + TBlLayoutChildNode}.
• There aren't any other users of those 3 traits

And it doesn't help to comprehend the code.

So, I propose to:

• Move all methods of TBlLayoutNode to BlLayoutNode
• Convert TBlLayoutChildNode to BlLayoutChildNode, and insert it as subclass of TBlLayoutNode and superclass of 4 classes (BlLayoutChildNode*)
• Same with TBlLayoutParentNode with all BlLayoutParentNode* classes