[bug][core] Refactor TextRange and TextDocument classes

[szabototo89]

We should refactor TextDocument and TextRange classes according to design ideas.

TextRange should not have Text property (or should not pass it), just Start, Stop and Length.

[szabototo89]

TextDocument should have the following properties:

interface ITextDocument {
  String Text { get; }
  TextRange TextRange { get; }  // returns TextRange of whole TextDocument
}

TextRange should have the following properties:

interface ITextRange {
  TextDocument TextDocument; // reference to original text document
  Int32 Start, Stop, Length;  // some information about the text
  String Text; // returns TextDocument.Text.SubString(Start, Stop);
  IEnumerable<ITextRange> Children  // Shouldn't be observable?
}

[szabototo89]

It is done.