WebSite X5 Evolution

What's new in this version:

- OpenCV 4.5.0 licence has been changed to Apache 2: OE-32 - Change OpenCV License to Apache 2
- OpenJPEG replaces Jasper in distributed Windows packages: #18194
- Added support for OpenCL multiple contexts

DNN module:
Improved layers / activations / supported more models:
- ONNX: ReduceSum, Gather, improved Reshape

Intel® Inference Engine backend ( OpenVINO™ ):
- added support for OpenVINO 2021.1 release

- Fixes and optimizations in DNN CUDA backend (thanks to @YashasSamaga): PRs

G-API module:
- Added Python bindings for the general G-API (#17493), inference and streaming support are currently in progress
- Introduced serialization API, currently limited. A graph can be serialized to a byte stream (std::vector) and then reconstructed back to a cv::GComputation object
- OpenVINO NN backend has been moved to the latest OpenVINO "Core" APIs
- Now it is easier to declare a graph with a dynamic number of inputs or outputs (GIn() / GOut() support operator+=)
- Added "mask", extended "convertTo" with 16S, also added performance tests on Core kernels for the Fluid backend
- Added standard NN post-processing operations like parseYolo and parseSSD to turn NN results into a structured graph-based data format (e.g. GArray)

Added value initialization for GArray so now one can write:
- cv::GArray arr = std::vector { 1, 2, 3 }
- to associate a cv::GArray<> with a value at graph construction stage
- cv::GMat bitwise operators are extended with a cv::GScalar operand
- Introduced a new graph data type cv::GFrame and its data storage type cv::MediaFrame to start handling media-oriented formats (like NV12 or YUV images with external memory) in the graph

And many other great contributions from OpenCV community:
- flann module fixes / improvements by @pemmanuelviel: PRs
- Add Robot-World/Hand-Eye calibration function

GSoC 2020 merged projects:
- GSoC: Better SIFT in the main repository
- GSoC: Improvement of Random Sample Consensus in OpenCV
- GSoC: Real-time Single Object Tracking using Deep Learning
- GSoC: Revise/improve Text and Digit Recognition Samples
- GSoC: Optimize OpenCV for RISC-V
- GSoC (opencv_contrib): OpenCV bindings for Julia
- GSoC (opencv_contrib): Implement Macbeth Chart detector and AprilTag3
- GSoC (opencv_contrib): Depth Fusion for large scale environments