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
Remove unnecessary kercn limitation of 4.

When accessing global memory by DWORD4, memory bandwidth
can be fully utilized on Intel platform. This patch will
make more image format(e.g. 8UC4) be processed in DWORD4
by work-item. After applying this patch, 3 subcase of
./opencv_perf_core --gtest_filter=OCL_RepeatFixture_Repeat.Repeat/*
can be speedup on HD4000 graphics card with Beignet:
OCL_RepeatFixture_Repeat.Repeat/2, 64% improvement.
OCL_RepeatFixture_Repeat.Repeat/6, 50% improvement.
OCL_RepeatFixture_Repeat.Repeat/8, 56% improvement.

Signed-off-by: Chuanbo Weng <chuanbo.weng@intel.com>
Fixed whitespace warnings in new tutorials
Reduce variables scope, move declaration to first use point
Merge pull request #3486 from Chuanbo-Weng:repeat-perf
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