fixing some wrong CPP prefixes - still old interface

Fix for the remarks in http://code.opencv.org/issues/3083 
- Changed the documentation compared to the loaded code snippets in the display_image section
- Changed the right commands, without CV_ in front, in the linux tutorial

For the prefixes it was still necessary to include
      #include <opencv2/highgui/highgui_c.h>
which basically seems a very wrong approach to me.