Merge pull request #15888 from OrestChura:facebeautification_gapi_sample

Merge pull request #15888 from OrestChura:facebeautification_gapi_sample

Introducing the sample of Face Beautification algorithm implemented via Graph-API

* Introducing the sample of Face Beautification algorithm implemented via Graph-API
- 'gapi/samples/face_beautification.cpp' added
- FIXME added in 'gcpukernel.hpp'

* INF_ENGINE fix
- preprocessing clauses added not to run the sample without Inference Engine

* INF_ENGINE fix 2
- warnings removed

* Fixes
- checking IE version cut as there is no dependency
- some alignments fixed
- the comment about preprocessing commands fixed

* ie::backend() issue fix (according to dmatveev)
- as the sample needs the cv::gapi::ie::backend() to be defined regardless of having IE or not, there is its throw-error definition in `giebackend.cpp` now (by dmatveev)
- for the same reason, #includes in `giebackend.hpp` are fixed
- HAVE_INF_ENGINE check is removed from the sample