types.hpp: Introduce NOM_STUBBED macro
Refactor JSON module

* Introduce nom::Value & supporting classes
* Combine nom::JSON::FileReader && nom::JSON::FileWriter into
nom::JsonSerializer.
* Introduce nom::JsonCppSerializer class
* Rename nom::JSON::Value class to nom::JsonCppValue
* Removal of json/config_json.hpp
* types.hpp: Introduce NOM_DASHED_ENDL macro
* Move NOM_STUBBED macro from types.hpp to config.hpp
examples/sprite_sheets: Use cursors for both input & output tests
SpriteSheet: Use nom::Value for file serialization
src/CMakeLists.txt: Add RenderWindow.cpp & GrayWindow.cpp

This corrects a mishaps with me forgetting to merge in some changes
from feature/GUI into feature/dev & feature/JSON until late -- I
think...