Merge pull request #12674 from dmatveev:gapi_upd270918

* Update G-API code base to 27-Sep-18

Changes mostly improve standalone build support

* G-API code base update 28-09-2018

* Windows/Documentation warnings should be fixed
* Fixed stability issues in Fluid backend
* Fixed precompiled headers issues in G-API source files

* G-API code base update 28-09-18 EOD

* Fixed several static analysis issues
* Fixed issues found when G-API is built in a standalone mode

modules/gapi/test/gapi_fluid_test_kernels.cpp

@@ -4,6 +4,8 @@
 //
 // Copyright (C) 2018 Intel Corporation
 
+#include "test_precomp.hpp"
+
 #include <iomanip>
 #include "gapi_fluid_test_kernels.hpp"
 #include <opencv2/gapi/core.hpp>