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/test/own/conc_queue_tests.cpp

@@ -55,7 +55,7 @@ TEST(ConcQueue, Clear)
     EXPECT_FALSE(q.try_pop(x));
 }
 
-// In this test, every writer thread produce its own range of integer
+// In this test, every writer thread produces its own range of integer
 // numbers, writing those to a shared queue.
 //
 // Every reader thread pops elements from the queue (until -1 is
@@ -64,12 +64,12 @@ TEST(ConcQueue, Clear)
 // Finally, the master thread waits for completion of all other
 // threads and verifies that all the necessary data is
 // produced/obtained.
+namespace
+{
 using StressParam = std::tuple<int           // Num writer threads
                               ,int           // Num elements per writer
                               ,int           // Num reader threads
                               ,std::size_t>; // Queue capacity
-namespace
-{
 constexpr int STOP_SIGN = -1;
 constexpr int BASE      = 1000;
 }