Merge pull request #16066 from dmatveev:dm/gapi_slides

* G-API: Added G-API Overview slides & its source code

- Sample code snippets are moved to separate files;
- Introduced a separate benchmark to measure Fluid/OpenCV
  performance;
- Added notes on API changes (it is still a 4.0, not a 4.2 talk!)
- Added a "Metropolis" beamer download-n-build script.

* G-API: Addressed review issues on G-API overview slides

modules/gapi/perf/cpu/gapi_core_perf_tests_cpu.cpp

@@ -288,4 +288,4 @@ INSTANTIATE_TEST_CASE_P(ResizeFxFyPerfTestCPU, ResizeFxFyPerfTest,
         Values(0.5, 0.1),
         Values(0.5, 0.1),
         Values(cv::compile_args(CORE_CPU))));
-}
+} // opencv_test