Some fixes for skeleton patch related with merge & delete, undo/redo

[bsekachev]

Motivation and context

Resolved #2157

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a 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.

[bsekachev]

/check

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[bsekachev]

@klakhov Could you please look at this patch carefully and try it?

[klakhov]

Merge & delete works fine with this patch for me.

Btw what do you think about making merge selection for sceletons more noticable like its done with box tracks?

[bsekachev]

Btw what do you think about making merge selection for sceletons more noticable like its done with box tracks?

@klakhov

I do not think it will work for skeletons, because they are not closed shapes.
We can only fill skeleton points to blue line like it is done for points shape.

[bsekachev]

We can only fill skeleton points to blue line like it is done for points shape.

@klakhov

I did so.

[klakhov]

Good job!