Doxygen. clearGroup,removeObject,closeFigure. Renaming of figure,clear,connect,disconnect in API

- New functions in C++ API and viewer: clearGroup, removeObject. Fixes #11 
- closeFigure in C++ API
- Renaming of functions in API for clarity:
  - figure -> newFigure
  - clear -> clearFigure
  - connect -> beginDrawing
  - disconnect -> endDrawing
- Doxygen comments in C++ API