G-API: Introduce streaming::desync and infer(ROI)

- desync() is a new (and for now, the only one) intrinsic
  which splits the graph execution into asynchronous parts
  when running in Streaming mode;
- desync() makes no effect when compiling in Traditional mode;
- Added tests on desync() working in various scenarios;
- Extended GStreamingExecutor to support desync(); also extended
  GStreamingCompiled() with a new version of pull() returning a
  vector of optional values;
- Fixed various issues with storing the type information & proper
  construction callbacks for GArray<> and GOpaque;

- Introduced a new infer(Roi,GMat) overload with a sample;

- Introduced an internal API for Islands to control fusion
  procedure (to fuse or not to fuse);
- Introduced handleStopStream() callback for island executables;
- Added GCompileArgs to metadata of the graph (required for other
  features).

modules/gapi/include/opencv2/gapi.hpp

@@ -33,4 +33,8 @@
 #include <opencv2/gapi/gkernel.hpp>
 #include <opencv2/gapi/operators.hpp>
 
+// Include this file here to avoid cyclic dependency between
+// Desync & GKernel & GComputation & GStreamingCompiled.
+#include <opencv2/gapi/streaming/desync.hpp>
+
 #endif // OPENCV_GAPI_HPP