If both Python 2 and Python 3 are found, then build bindings for both of
them during the build process.  Currently, one version of Python is
detected automatically, and building for the other requires changes the
CMake config.

The largest chunk of this change generalizes OpenCVDetectPython.cmake to
find both a Python 2 and Python 3 version of Python.  Secondly, the
opencv_python module is split into two modules, opencv_python2 and
opencv_python3.  Both are built from the same source. but for different
versions of Python.

put the detection-based tracker back (to objdetect module), so that the Android face detection sample builds fine. The patch by Konstantin Matskevich that adds C++11 threads support has been merged in too. Also, fixed compile error in the doc builder.

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.

The given ORB parameter was misspelled

Fixed core for CCORR and SQDIFF. Used float instead of int for CV_8U. Fixed conditions for call dft.

binary mode

modules/core/src/arithm.cpp:345:51:
error: constant expression evaluates to 4294967295 which cannot be narrowed to type 'int' [-Wc++11-narrowing]
static int CV_DECL_ALIGNED(16) v64f_absmask[] = { 0xffffffff, 0x7fffffff, 0xffffffff, 0x7fffffff };
                                                  ^~~~~~~~~~

Converted to unsigned int.  This variable is only used to initialize a bit pattern anywhy for a 128bit type.