Suppressed hundreds of useless MSVC warnings (can be reenabled setting cmake variable ENABLE_NOISY_WARNINGS to ON). Fixed some of remaining warnings.

samples/cpp/CMakeLists.txt

@@ -20,7 +20,7 @@ if(BUILD_EXAMPLES AND OCV_DEPENDENCIES_FOUND)
     ocv_include_directories("${OpenCV_SOURCE_DIR}/modules/gpu/include")
   endif()
 
-  if(CMAKE_COMPILER_IS_GNUCXX)
+  if(CMAKE_COMPILER_IS_GNUCXX AND NOT ENABLE_NOISY_WARNINGS)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-function")
   endif()