cmake: refactored scripts with samples building:

- allow installing samples sources on all platforms
  even if BUILD_EXAMPLES is disabled,  fixed minor
  issues in sources installation process
- use 'example_<group>_<name>' scheme for target and binary file naming
- use single function for sample executable creation

samples/cpp/tutorial_code/snippets/imgproc_HoughLinesPointSet.cpp

@@ -1,4 +1,5 @@
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
 
 using namespace cv;
 using namespace std;