G-API-NG/Streaming: don't require explicit metadata in compileStreaming()

* First probably working version
Hardcode gose to setSource() :)

* Pre final version of move metadata declaration from compileStreaming() to setSource().

* G-API-NG/Streaming: recovered the existing Streaming functionality

- The auto-meta test is disabling since it crashes.
- Restored .gitignore

* G-API-NG/Streaming: Made the meta-less compileStreaming() work

- Works fine even with OpenCV backend;
- Fluid doesn't support such kind of compilation so far - to be fixed

* G-API-NG/Streaming: Fix Fluid to support meta-less compilation

- Introduced a notion of metadata-sensitive passes and slightly
  refactored GCompiler and GFluidBackend to support that
- Fixed a TwoVideoSourcesFail test on streaming

* Add three smoke streaming tests to gapi_streaming_tests.
All three teste run pipeline with two different input sets
1) SmokeTest_Two_Const_Mats test run pipeline with two const Mats
2) SmokeTest_One_Video_One_Const_Scalar test run pipleline with Mat(video source) and const Scalar
3) SmokeTest_One_Video_One_Const_Vector test run pipeline with Mat(video source) and const Vector
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* style fix

* Some review stuff

* Some review stuff