Added a new TextDocument class. It represents raw text of source code. Its node are the text nodes which are shallow versions of tree.
CodeSharper.Core/Texts/TextDocument:
  - Added a new class which is the root of text tree
  - Moved AppendChild, RemoveChild and Children methods from TextNode
CodeSharper.Core/Texts/TextNode:
  - Removed Children, AppendChild and RemoveChild methods
  - Changed type of Parent from TextNode to TextDocument