Doxygen tutorials: some fixes in RST-docs
Doxygen tutorials: basic structure
Doxygen tutorials: warnings cleared
Doxygen tutorials: cpp done
Doxygen tutorials: python basic
Doxygen tutorials: python final edits
User guide converted to doxygen
Doxygen tutorials: support contrib, some minor changes
Documentation: removed mention of Mat(IplImage*) constructor
Fixed whitespace warnings in new tutorials
Reduce variables scope, move declaration to first use point
Fix build for Android arm64-v8a
Merge pull request #3492 from Dmitry-Me:reduceVariablesScope1
Merge pull request #3479 from mshabunin:doxygen-tutorials
Merge pull request #3495 from taka-no-me:fix_android_arm64
cap_ffmpeg: drop support for very old libav* versions

All relevant distros now ship the equivalent of Libav 0.8 or newer, so
drop support for everything older as it is a maintenance burden.
cap_ffmpeg: drop the local copy of the RIFF FourCC list

It is now exported by libavformat through a public API, so there is no
reason to carry an outdated custom copy.
cap_ffmpeg: wrap a forgotten instance of CODEC_ID_H264 in CV_CODEC()
cap_ffmpeg: do not use AVStream.r_frame_rate

r_frame_rate is a guessed number defined as "the least common multiple
of all framerates in the stream". It has been deprecated and removed in
new Libav releases.

Use avg_frame_rate instead, which is the average framerate of the
stream.
cap_ffmpeg: use avcodec_encode_video2() where available

avcodec_encode_video() has been deprecated and removed in newer
libavcodec versions.