* G-API: Introduce a new gapi::infer2 overload + gaze estimation sample

* G-API/infer2: Introduced static type checking for infer2

- Also added extra tests on the type check routine

* G-API/infer2: Addressed self-review comments in the sample app

- Also fix build on Linux;

* G-API/infer2: Remove incorrect SetLayout(HWC) + dead code

- Also fixed comments in the backend

* G-API/infer2: Continue with self-review

- Fix warnings/compile errors in gaze estimation
- Dropped the use of RTTI (VectorRef::holds()) from the giebackend
- Replaced it with a trait-based enums for GArray<T> and std::vector<T>
- The enums and traits are temporary and need to be unified with
  the S11N when it comes

* G-API/infer2: Final self-review items

- Refactored ROIList test to cover 70% for infer<> and infer2<>;
- Fixed the model data discovery routine to be compatible with new
  OpenVINO;
- Hopefully fixed the final issues (warnings) with the sample.

* G-API/infer2: address review problems

- Fixed typo in comments;
- Fixed public (Doxygen) comment on GArray<GMat> input case for infer2;
- Made model lookup more flexible to allow new & old OMZ dir layouts.

* G-API/infer2: Change the model paths again

* G-API/infer2: Change the lookup path for test data

* G-API/infer2: use randu instead of imread. CI war is over