Merge pull request #18182 from OrestChura:oc/operators_to_cv

[G-API]: Relocation of operators' overloads

* Relocates overloaded operators for `cv::GMat` and `cv::GScalar` to `cv::` namespace
 - adds test to check usage of operators compilation

* Add tests for all the operators

* Address comments

modules/gapi/src/api/operators.cpp

@@ -12,6 +12,8 @@
 #include <opencv2/gapi/gscalar.hpp>
 #include <opencv2/gapi/operators.hpp>
 
+namespace cv
+{
 cv::GMat operator+(const cv::GMat& lhs, const cv::GMat& rhs)
 {
     return cv::gapi::add(lhs, rhs);
@@ -211,3 +213,4 @@ cv::GMat operator!=(const cv::GScalar& lhs, const cv::GMat& rhs)
 {
     return cv::gapi::cmpNE(rhs, lhs);
 }
+} // cv