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
* Update G-API code base to 27-Sep-18
Changes mostly improve standalone build support
* G-API code base update 28-09-2018
* Windows/Documentation warnings should be fixed
* Fixed stability issues in Fluid backend
* Fixed precompiled headers issues in G-API source files
* G-API code base update 28-09-18 EOD
* Fixed several static analysis issues
* Fixed issues found when G-API is built in a standalone mode
* 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
