(2.4) Backports from master branch (#7854)

/usr/lib/python3.5/re.py:203: FutureWarning: split() requires a non-empty pattern match.
  return _compile(pattern, flags).split(string, maxsplit)

 - cv2.UMat implemented - python thin wrapper for UMat
 - no implicit copy from GPU to Host done, resulting UMat can be passed to next function without overhead
 - cv2.UMat.get() - to fetch data to Host
 - new tests covers: ORB, BFMatcher, goodFeaturesToTrack, calcOpticalFlowPyrLK

don't discard pure virtual methods in hdr_parser.py (was broken anyway)

maintain set of visited namespaces in CppHeaderParser
WARNING: REMOVED const name conversion (SomeConstName -> SOME_CONST_NAME), discussion needed

gen2.py reports headers it processes
cmake file restructured

removed contrib, legacy and softcsscade modules; removed latentsvm and datamatrix detector from objdetect. removed haartraining and sft apps.
some of the stuff will be moved to opencv_contrib module.
in order to make this PR pass buildbot, please, comment off opencv_legacy, opencv_contrib and opencv_softcascade test runs.

Also, removed the one from modules/python/src2/cv.py and cleared its
executable bit, since it's not a script.

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

* always use "argN" names for unnamed arguments
* honor space symbol between typename and "*", "&" symbols
* fix indent errors

adjust the header parser to support "public virtual" construction and make a tweak for multiple inheritance, fix potential memory problem with Python's IplImage.tostring() method (ticket #1486)

updated feature_homography.py sample to use new features

upgraded to FLANN 1.6. Added miniflann interface, which is now used in the rest of OpenCV. Added Python bindings for FLANN.