samples: gpu: removed softcascade example

The example in question cannot be compiled anymore as the softcascade
module has been removed in 3858f2291d64652a82fea930ba5c598045633843.
samples: gpu: removed inclusion of non-existent opencv2/contrib/contrib.hpp header, re-introduced TickMeter class in a separate header

This patch removes inclusion of opencv2/contrib/contrib.hpp header,
which does not exist anymore due to removal of opencv_contrib module.

The samples including this header appear to be doing so in order to
use TickMeter class; therefore, the latter is now provided by tick_meter.hpp
header file, located in samples/gpu folder.
samples: gpu: performance: remove inclusion of opencv2/legacy.hpp header

The header is gone now, along with the opencv_legacy module.
samples: gpu: performance: removed FGDStatModel performance comparison test

The FGD background subtraction model was part of opencv_legacy module;
as it is gone now, it makes little sense to benchmark the GPU version,
either, so the whole test is removed.
samples: gpu: super_resolution: disable simple optical flow

The cv::superres::createOptFlow_Simple() function along with the
simple optical flow class implementation is currently commented out
in the superres module's code, so comment it out in the example as well.
samples: gpu: performance: fix include path for opencv_xfeatures2d

As opencv_xfeatures2d is part of opencv-contrib and not opencv repository,
${OpenCV_SOURCE_DIR}/modules/modules/include is not a correct include
path - use ${opencv_xfeatures2d_SOURCE_DIR}/include instead
samples: gpu: performance: BackgroundSubtractorMOG is now part of opencv_bgsegm module
samples: gpu: performance: SURF is now part of opencv_xfeatures2d
updated links to the official OpenCV youtube channel
remove structured_edge_detection.hpp from blacklist
don't discard pure virtual methods in hdr_parser.py (was broken anyway)
Merge pull request #3179 from alekcac:update_in_youtube_url
Merge pull request #3169 from znah:py_wrap
Changed check to NORM_RELATIVE for OCL.Mat_Scalar_Scale tests
Merge pull request #3181 from akarsakov:ocl_mul_scale_test_fix
Merge pull request #3174 from rokm:samples-fix
adding fourcc to VideoWriter docs