G-API external backend development (#13943)

* Moved HostCtor and ConstVal from gapi_priv.hpp to objref.hpp

* Added gmodel_priv.hpp, added export of symbols from gmodel.hpp

* Added export of binInArg and bindOutArg

* Renamed gapi_priv.*pp -> gorigin.*pp

* Added a fixme on collecting exports inside one class

* G-API Initial code upload

* Update G-API code base to Sep-24-2018

* The majority of OpenCV buildbot problems was addressed

* Update G-API code base to 24-Sep-18 EOD

* G-API code base update 25-Sep-2018

* Linux warnings should be resolved
* Documentation build should become green
* Number of Windows warnings should be reduced

* Update G-API code base to 25-Sep-18 EOD

* ARMv7 build issue should be resolved
* ADE is bumped to latest version and should fix Clang builds for macOS/iOS
* Remaining Windows warnings should be resolved
* New Linux32 / ARMv7 warnings should be resolved

* G-API code base update 25-Sep-2018-EOD2

* Final Windows warnings should be resolved now

* G-API code base update 26-Sep-2018

* Fixed issues with precompiled headers in module and its tests