Add new labels while labelling

This was asked in #2403, but there have been no responses. For exploring a new dataset where the actual labels are unknown from the start, human annotators should be able to add new labels during the annotation process.

For example, a set of pre-defined labels with an add label button/text box to add labels during the annotation.

Additionally, it would be great to have a textbox for additional textual comments on each task, such as reporting issues with the file or adding specific information.