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/src/api/ginfer.cpp

@@ -0,0 +1,27 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+//
+// Copyright (C) 2018-2019 Intel Corporation
+
+
+#include "precomp.hpp"
+
+#include <functional> // hash
+#include <numeric> // accumulate
+#include <unordered_set>
+#include <iterator>
+
+#include <ade/util/algorithm.hpp>
+
+#include <opencv2/gapi/infer.hpp>
+
+cv::gapi::GNetPackage::GNetPackage(std::initializer_list<GNetParam> &&ii)
+    : networks(std::move(ii)) {
+}
+
+std::vector<cv::gapi::GBackend> cv::gapi::GNetPackage::backends() const {
+    std::unordered_set<cv::gapi::GBackend> unique_set;
+    for (const auto &nn : networks) unique_set.insert(nn.backend);
+    return std::vector<cv::gapi::GBackend>(unique_set.begin(), unique_set.end());
+}