ocl: split initialization.cpp into 3 files: context, operations, programcache
ocl: move program names into opencl_kernels.hpp
ocl: file-based ProgramCache refactoring
ocl: added OpenCL device selection via OPENCV_OPENCL_DEVICE environment variable
ocl: merge with upstream/2.4
ocl: workaround for OpenCL C++ bindings usage: CL/cl.hpp
ocl: update ocl samples
ocl: update module documentation
Merge pull request #1561 from alalek:ocl_refactoring