Add default colors for masks

[zhiltsov-max]

• Added default color table for mask export
• Added an option to use the source colormap in VOC segmentation export when it's provided with source colormap mode
• Removed label name changing when exporting masks

Motivation and context

When there are tasks with the same labels, possibly, in different order, it may be useful and is often expected that these tasks will have the same colors for the same labels. This patch adds this functionality by adding a default color table for typical labels.

How has this been tested?

Unit tests.

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

[coveralls]

Pull Request Test Coverage Report for Build 5415

• 52 of 58 (89.66%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 65.737%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/dataset_manager/formats/mask.py	30	33	90.91%
datumaro/datumaro/plugins/voc_format/converter.py	22	25	88.0%

Totals
Change from base Build 5409:	0.09%
Covered Lines:	10876
Relevant Lines:	16144

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💛 - Coveralls