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

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

Made all STL usages explicit to be able automatically find all usages of
particular class or function.

This reverts commit 4aaaef59.

Conflicts:
    modules/imgproc/include/opencv2/imgproc/types_c.h
    modules/legacy/include/opencv2/legacy/legacy.hpp

put template Distance classes back to features2d, since they are used in GPU; fixed OpenCV build with Xcode 2.3 (which includes new libpng)

added BruteForceMatcher (the older variant of BFMatcher) to legacy (ticket #1796). added test for it. Renamed legacy tests to "Legacy_*"

added dedicated <modname>_init.cpp files with initModule_<modname>() functions and all the relevant structures; made BackgroundSubtractorMOG/MOG2 derivatives from Algorithm; cleaned up MOG2 implementation and made it multi-threaded.

lot's of changes; nonfree & photo modules added; SIFT & SURF -> nonfree module; Inpainting -> photo; refactored features2d (ORB is still failing tests), optimized brute-force matcher and made it non-template.

moved some old stuff to the legacy module; merge "compat_c.h" headers and moved to the legacy as well. moved implementation of many non-critical/obsolete inline functions and methods to .cpp to improve Opencv build time