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

fix wrong conversion in estimateAffine2D* functions

Wrappers for load methods of EM, LR, SVMSGD and Normal Bayes Classifier