Merge pull request opencv#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/doc/slides/.gitignore

@@ -0,0 +1,6 @@
+*.bbl
+*.blg
+*.sty
+*.tex
+*-converted-to.pdf
+mtheme.sty/

modules/gapi/doc/slides/README.md

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+# G-API Overview
+
+This is the latest overview slide deck on G-API.
+
+## Prerequisites
+
+- [Emacs] v24 or higher;
+- [Org]-mode 8.2.10;
+- `pdflatex`;
+- `texlive-latex-recommended` ([Beamer] package);
+- `texlive-font-utils` (`epstopdf`);
+- `wget` (for `get_sty.sh`).
+
+## Building
+
+1. Download and build the [Metropolis] theme with the script:
+
+```
+$ ./get_sty.sh
+```
+
+2. Now open `gapi_overview.org` with Emacs and press `C-c C-e l P`.
+
+[Emacs]:      https://www.gnu.org/software/emacs/
+[Org]:        https://orgmode.org/
+[Beamer]:     https://ctan.org/pkg/beamer
+[Metropolis]: https://github.com/matze/mtheme