cv2.findContours() no longer modifies source image (#8150)

* cv2.findContours() no longer modifies source image

Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.  ??? Do I have that correct ???

Also fixed up the code blocks to be valid markdown.

* ```python --> @code{.py}

Enables syntax highlighting in docs.

Solve issue 8136 opencv doc of threshold

Add a link to MatExpr in Detailed Description of Mat

The backslash causes build failure when cross-compiling with mingw-w64

fix for opencv/opencv#8105, compilation issue with mingw32 (in
google/googletest#721 a similar issue was solved and the reason was
described as MinGW defines _CRITICAL_SECTION and _RTL_CRITICAL_SECTION
as two separate (equivalent) structs, instead of using typedef)

CoInitialize/CoUninitialize in VFW classes (#8112)

- fixed returned compactness value
- added centers drawing to the example app
- added compactness test

 - brush up v_reduce_sum of SSE version

Add wrappers for load functions for DTrees and Boost classifiers