Merge pull request #12608 from dmatveev:gapi

* G-API Initial code upload

* Update G-API code base to Sep-24-2018

* The majority of OpenCV buildbot problems was addressed

* Update G-API code base to 24-Sep-18 EOD

* G-API code base update 25-Sep-2018

* Linux warnings should be resolved
* Documentation build should become green
* Number of Windows warnings should be reduced

* Update G-API code base to 25-Sep-18 EOD

* ARMv7 build issue should be resolved
* ADE is bumped to latest version and should fix Clang builds for macOS/iOS
* Remaining Windows warnings should be resolved
* New Linux32 / ARMv7 warnings should be resolved

* G-API code base update 25-Sep-2018-EOD2

* Final Windows warnings should be resolved now

* G-API code base update 26-Sep-2018

* Fixed issues with precompiled headers in module and its tests

modules/gapi/src/api/gnode.cpp

@@ -0,0 +1,88 @@
+// 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 Intel Corporation
+
+
+#include <cassert>
+
+#include "api/gnode.hpp"
+#include "api/gnode_priv.hpp"
+
+// GNode private implementation
+cv::GNode::Priv::Priv()
+    : m_shape(NodeShape::EMPTY)
+{
+}
+
+cv::GNode::Priv::Priv(GCall c)
+    : m_shape(NodeShape::CALL), m_spec(c)
+{
+}
+
+cv::GNode::Priv::Priv(ParamTag)
+    : m_shape(NodeShape::PARAM)
+{
+}
+
+cv::GNode::Priv::Priv(ConstTag)
+    : m_shape(NodeShape::CONST_BOUNDED)
+{
+}
+
+// GNode public implementation
+cv::GNode::GNode()
+    : m_priv(new Priv())
+{
+}
+
+cv::GNode::GNode(const GCall &c)
+    : m_priv(new Priv(c))
+{
+}
+
+cv::GNode::GNode(ParamTag)
+    : m_priv(new Priv(Priv::ParamTag()))
+{
+}
+
+cv::GNode::GNode(ConstTag)
+    : m_priv(new Priv(Priv::ConstTag()))
+{
+}
+
+cv::GNode cv::GNode::Call(const GCall &c)
+{
+    return GNode(c);
+}
+
+cv::GNode cv::GNode::Param()
+{
+    return GNode(ParamTag());
+}
+
+cv::GNode cv::GNode::Const()
+{
+    return GNode(ConstTag());
+}
+
+cv::GNode::Priv& cv::GNode::priv()
+{
+    return *m_priv;
+}
+
+const cv::GNode::Priv& cv::GNode::priv() const
+{
+    return *m_priv;
+}
+
+const cv::GNode::NodeShape& cv::GNode::shape() const
+{
+    return m_priv->m_shape;
+}
+
+const cv::GCall& cv::GNode::call()  const
+{
+    return util::get<GCall>(m_priv->m_spec);
+}