Added ability to search for an empty frame

[bsekachev]

Motivation and context

Added ability to search empty frames.
The request is from our data annotation team.

Added switcher to player buttons next/previous that supports three kinds of navigation:

• regular navigation (as it was)
• navigation using filter search (as it was implemented using arrows)
• navigation till the nearest empty frame

Right click to a current navigation button opens switcher, then you can use other kind of navigation clicking it. It will change default behavior (including shortcuts)

Resolve #2214
Resolve #284

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

[coveralls]

Coverage decreased (-0.5%) to 64.858% when pulling 9d86b3e on bs/empty_frames_search into d4b0e11 on develop.

[Loc-Vo]

Can we also have the statistics of empty frames included in the INFO panel for status tracking as well?

[bsekachev]

@s3298230

Can we also have the statistics of empty frames included in the INFO panel for status tracking as well?

Please, submit a github issue. It looks like not difficult, but separated feature.