Big OpenCV Cleanup

[vpisarev]

Describe the feature and motivation

We need to clean OpenCV 5.0 codebase, especially the main repository, in order to create room for new functionality and reduce maintenance effort.

This is the master ticket that refers to several child topics:

• Use standard C++ types in API
• Clean Core module
• Clean Imgproc module
• Clean Features2D, rename it to Features
• Replace FLANN with new smaller and better implementation
• Clean Objdetect module
• Move ML module to opencv_contrib
• Remove OpenVX
• Move IPP to opencv/3rdparty
• Move G-API to opencv_contrib or to the separate opencv/G-API repository
• Clean Samples

Additional context

No response

[s-trinh]

Maybe pin also this ticket to avoid it being submerged by issues? And keep it visible?

Just saw that there is also https://github.com/opencv/opencv/projects?query=is%3Aopen but I am not used to it.