Minimize usages of legacy C API inside the library

doc/tutorials/core/adding_images/adding_images.rst

@@ -35,8 +35,7 @@ As usual, after the not-so-lengthy explanation, let's go to the code:
 
 .. code-block:: cpp
 
-   #include <cv.h>
-   #include <highgui.h>
+   #include <opencv2/opencv.hpp>
    #include <iostream>
 
    using namespace cv;

doc/tutorials/core/basic_linear_transform/basic_linear_transform.rst

@@ -72,8 +72,7 @@ Code
 
 .. code-block:: cpp
 
-   #include <cv.h>
-   #include <highgui.h>
+   #include <opencv2/opencv.hpp>
    #include <iostream>
 
    using namespace cv;