[G-API]: Add GArray initialization support
* Added GArray initialization (CONST_VALUE, GScalar analog) and test for this
* Whitespaces
* And one more space
* Trailing whitespace
* Test name changed. Build with magic commands.
* GArray works with rvalue initialization
* Code cleanup
* Ternary operator in the initialization list.
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