Merge pull request #13030 from dmatveev:tutorial

* G-API: First steps with tutorial

* G-API Tutorial: First iteration

* G-API port of anisotropic image segmentation tutorial;
* Currently works via OpenCV only;
* Some new kernels have been required.

* G-API Tutorial: added chapters on execution code, inspection, and profiling

* G-API Tutorial: make Fluid kernel headers public

For some reason, these headers were not moved to the public
headers subtree during the initial development. Somehow it even
worked for the existing workloads.

* G-API Tutorial: Fix a couple of issues found during the work

* Introduced Phase & Sqrt kernels, OCV & Fluid versions
* Extended GKernelPackage to allow kernel removal & policies on include()

All the above stuff needs to be tested, tests will be added later

* G-API Tutorial: added chapter on running Fluid backend

* G-API Tutorial: fix a number of issues in the text

* G-API Tutorial - some final updates

- Fixed post-merge issues after Sobel kernel renaming;
- Simplified G-API code a little bit;
- Put a conclusion note in text.

* G-API Tutorial - fix build issues in test/perf targets

Public headers were refactored but tests suites were not updated in time

* G-API Tutorial: Added tests & reference docs on new kernels

* Phase
* Sqrt

* G-API Tutorial: added link to the tutorial from the main module doc

* G-API Tutorial: Added tests on new GKernelPackage functionality

* G-API Tutorial: Extended InRange tests to cover 32F

* G-API Tutorial: Misc fixes

* Avoid building examples when gapi module is not there
* Added a volatile API disclaimer to G-API root documentation page

* G-API Tutorial: Fix perf tests build issue

This change came from master where Fluid kernels are still used
incorrectly.

* G-API Tutorial: Fixed channels support in Sqrt/Phase fluid kernels

Extended tests to cover this case

* G-API Tutorial: Fix text problems found on team review

modules/gapi/doc/00-root.markdown

@@ -12,6 +12,10 @@ specific CV algorithm. G-API provides means to define CV operations,
 construct graphs (in form of expressions) using it, and finally
 implement and run the operations for a particular backend.
 
+@note G-API is a new module and now is in active development. It's API
+is volatile at the moment and there may be minor but
+compatibility-breaking changes in the future.
+
 # Contents
 
 G-API documentation is organized into the following chapters:
@@ -103,7 +107,7 @@ There is a number important concepts can be outlines with this examle:
 
 <!-- FIXME: The above operator|() link links to MatExpr not GAPI -->
 
-See Tutorial[TBD] and Porting examples[TBD] to learn more on various
-G-API features and concepts.
+See [tutorials and porting examples](@ref tutorial_table_of_content_gapi)
+to learn more on various G-API features and concepts.
 
 <!-- TODO Add chapter on declaration, compilation, execution -->