Transfer SIFT to the main repository

[ychechik]

As of the day after tomorrow (7.3.20) the patent on SIFT will expire and so it should be free to use.

Moving it out of the non-free folder will enable all to compile opencv with default flags and still get SIFT (will also help for the python binding as it doesn't compile the non-free folder).

[vpisarev]

See https://github.com/opencv/opencv/wiki/GSoC_2020#idea-better-sift-in-the-main-repository

[alalek]

As a quick step in this way we can remove #ifdef guards with OPENCV_ENABLE_NONFREE for SIFT source code in opencv_contrib repository (without moving the code).

[saskatchewancatch]

@alalek @vpisarev , I attempted this in PR 2449 in opencv_contrib repository. It implements the "quick step" by moving SIFT out of non-free into free. I suppose the rest of the work involving moving it into opencv could be a later work for GSoC as per Vadim's suggestion.

[flokixdev]

Can I ask, when we can expect it to be included in a new release?
Thank you!

[alalek]

#17119 backlog:

• 2 disabled SIFT tests
• fix C++ compatibility? cv::xfeatures2d::SIFT::create() (problem is due MSVS ambiguous errors)
• fix Java bindings compatibility? org.opencv.features2d.SIFT.create()