Basic doxygen documentation support

- updated existing Doxyfile.in
- added corresponding cmake instructions
- added some specific files (layout, icon)
- clean existing doxygen warnings
Fixed imgcodecs tests

- Imgcodecs_Image, write_imageseq - assuming JPEG images have losses
- Imgcodecs_Drawing, c_regression - replaced assert calls
- Imgcodecs_Drawing - exact comparison with reference image
Disabled videoio_video_prop_fps test

This test does not return correct values with some software configurations.
Possible problem with ffmpeg on buildbot's linux-slave.
Try to optimize GRAY2RGB
Merge pull request #3361 from mshabunin:fix-drawing
Merge pull request #3343 from mshabunin:doxygen-docs
Merge branch 'master' of https://github.com/Itseez/opencv