[core] Refactor TextDocument to universal interface

[szabototo89]

There is a design problem with TextDocument. We considered this class and TextRange as a stable dependency, but we realized that changing implementation according to source code would be more efficient. It implicates that TextRange class should be refactored as an inner datastructure of TextDocument and it would be changed to Markers.

Markers would be identifiers of specific regions and totally implemantation independent.