Update G-API slides to OpenCV 4.4
* G-API: Updated slides to v4.4 (+ sample)
* Slight formatting changes + Python API page
* Some more updates to slides:
- Added more info on 4.2 and 4.4 versions
- Added explanation on Operations and their functional wrappers
* 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