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

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