WebSite X5 Evolution

What's new in this version:

- SIFT (Scale-Invariant Feature Transform) algorithm has been moved to the main repository (patent on SIFT is expired)

DNN module:
Improved layers / activations / supported more models:
- State-of-art Yolo v4 Detector: #17148. Disabled per-layer NMS for [yolo] layers (Yolo v3 and Yolo v4) as incorrect - use cv::dnn::NMSBoxes over all the detections
- ONNX: Add support for Resnet_backbone (Torchvision)

EfficientDet models support:
New samples / demos:
- add text recognition sample: C++ / Python

FlowNet2 optical flow:
Intel® Inference Engine backend ( OpenVINO™ ):
- added support for OpenVINO 2020.3 LTS / 2020.4 releases
- support of NN Builder API is planned for removal in the next release
- Many fixes and optimizations in CUDA backend: PRs

G-API module:
- Introduced a new API for stateful kernels in OpenCV backend: GAPI_OCV_KERNEL_ST. Stateful kernels preserve their state among the individual graph executions (in the standard more) or among the video frames of the stream (in the streaming mode)
- Introduced more video-oriented operations in G-API: goodFeaturesToTrack, buildOpticalFlowPyramid, calcOpicalFlowPyrLK
- Added few more image processing kernels: Laplacian and bilateral filters
- Fixed a potential crash in G-API's OpenCL backend

And many other great contributions from OpenCV community:
- Obj-C / Swift bindings
- (opencv_contrib) Julia bindings as part of ongoing GSoC project
- (opencv_contrib) BIMEF: A Bio-Inspired Multi-Exposure Fusion Framework for Low-light Image Enhancement
- Enable Otsu thresholding for CV_16UC1 images
- Add Stroke Width Transform algorithm for Text Detection
- Planned migration on Apache 2 license for next releases OE-32