Allow filtering for data without labels

[IgorSusmelj]

What has changed and why?

TL;DR

• we add a new virtual annotation class for "no annotation". We don't modify the db but create the virtual class in the backend to preserve the filtering logic. The user can now filter in the Python UI or frontend for "no annotations"

images-without-annotations.mp4

The changes in detail:

• Update filtering logic in backend code for images, frames and video
• Update the backend tests accoringly
• Add a new "checkbox" for the "No annotations" filter in the left filtering menu
• Handling of edge cases
  • We don't show the "No annotations" in annotation grid view
  • We don't show the "No annotations" if there are no annotations in the dataset

How has it been tested?

• Updated unit tests
• Manual tests with our default image and video datasets

Did you update CHANGELOG.md?

• Yes
• Not needed (internal change)

[IgorSusmelj]

/review

[IgorSusmelj]

Do not review this PR. We split it up into smaller PRs.