Core: SubTextNode should extend from TextNode

[szabototo89]

SubTextNode should be a logical subnode to TextNode.

public class SubTextNode : TextNode {
  public SubTextNode(TextRange range, TextNode parent) { /* ... */ }
  // Text is a reference to a substring of Parent.Text
  public override string Text {
     get { return Parent.Text.SubString(range.Start, range.End); }
     set { Parent.Text.SubString(range.Start, range.End) = value; }
  }
}

[szabototo89]

It is refactored to TextDocument class.