Intelligent polygon editing

[bsekachev]

Motivation and context

Our internal team and the community asked us to simplify polygon editing to remove extra action, related with selection of a final polygon.

This PR introduces updated approach. Two criteria are computer to decide which part of a polygon should be cut during editing automatically. The first criteria is a number of cut points. The second criteria is a length of a cut curve. If both criteria recommend to cut the same part, algorithm works automatically, if not, a user asked to make a choice.

How has this been tested?

Manual testing

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

Resolve #1579
Resolve #1515

[coveralls]

Pull Request Test Coverage Report for Build 6531

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 64.984%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	1	77.29%

Totals
Change from base Build 6516:	-0.004%
Covered Lines:	11064
Relevant Lines:	16620

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💛 - Coveralls

[rodo-qatar]

Is there any possibility to switch between the Intelligent polygon editing approach and the extra action approach?
I found specially annoying that I dont know in advance which part will be kept when splitting a polygon under the inteligent approach. Could this be disabled at request somehow?