Merge pull request #15090 from dmatveev:dm/ng-0001-g-api-inference-api

* G-API-NG/API: Introduced inference API and IE-based backend

- Very quick-n-dirty implementation
- OpenCV's own DNN module is not used
- No tests so far

* G-API-NG/IE: Refined IE backend, added more tests

* G-API-NG/IE: Fixed various CI warnings & build issues + tests

- Added tests on multi-dimensional own::Mat
- Added tests on GMatDesc with dimensions
- Documentation on infer.hpp
- Fixed more warnings + added a ROI list test
- Fix descr_of clash for vector<Mat> & standalone mode
- Fix build issue with gcc-4.8x
- Addressed review comments

* G-API-NG/IE: Addressed review comments

- Pass `false` to findDataFile()
- Add deprecation warning suppression macros for IE

modules/gapi/test/test_precomp.hpp

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include <opencv2/ts.hpp>
+
 #include <opencv2/gapi.hpp>
 #include <opencv2/gapi/imgproc.hpp>
 #include <opencv2/gapi/core.hpp>
@@ -25,5 +26,6 @@
 #include <opencv2/gapi/operators.hpp>
 #include <opencv2/gapi/fluid/imgproc.hpp>
 #include <opencv2/gapi/fluid/core.hpp>
+#include <opencv2/gapi/infer.hpp>
 
 #endif // __OPENCV_GAPI_TEST_PRECOMP_HPP__