Get list of subsets and support only Image media in visualizer

[sooahleex]

Summary

• Support list of subsets to show data of all subsets
• Support only Image media type in Visualizer
  • For brats_dataset, MultiframeImage media type is used

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Base: 84.64% // Head: 84.20% // Decreases project coverage by -0.44% ⚠️

Coverage data is based on head (06a27e0) compared to base (c5327b3).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #768      +/-   ##
===========================================
- Coverage    84.64%   84.20%   -0.45%     
===========================================
  Files          157      157              
  Lines        21925    21931       +6     
  Branches      4804     4806       +2     
===========================================
- Hits         18558    18466      -92     
- Misses        2263     2369     +106     
+ Partials      1104     1096       -8     

Flag	Coverage Δ
macos-10.15_Python-3.10	84.11% <50.00%> (?)
macos-10.15_Python-3.7	83.79% <50.00%> (-0.02%)	⬇️
macos-10.15_Python-3.8	84.03% <50.00%> (-0.46%)	⬇️
macos-10.15_Python-3.9	84.05% <50.00%> (-0.02%)	⬇️
ubuntu-20.04_Python-3.10	84.13% <50.00%> (+<0.01%)	⬆️
ubuntu-20.04_Python-3.7	83.79% <50.00%> (-0.02%)	⬇️
ubuntu-20.04_Python-3.8	84.03% <50.00%> (-0.02%)	⬇️
ubuntu-20.04_Python-3.9	84.05% <50.00%> (-0.02%)	⬇️
windows-2019_Python-3.10	84.13% <50.00%> (?)
windows-2019_Python-3.7	83.81% <50.00%> (-0.02%)	⬇️
windows-2019_Python-3.8	84.04% <50.00%> (?)
windows-2019_Python-3.9	84.06% <50.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/components/visualizer.py	92.89% <50.00%> (-1.88%)	⬇️
datumaro/plugins/synthetic_data/utils.py	17.94% <0.00%> (-64.11%)	⬇️
datumaro/plugins/synthetic_data/image_generator.py	69.93% <0.00%> (-12.89%)	⬇️
datumaro/components/operations.py	84.40% <0.00%> (+0.17%)	⬆️

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[vinnamkim]

LGTM.