* 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