Datasets tab for sketch editor

[daemontus]

Datasets represent various types of observations. Here, we can import these from tabular data formats and label/group them. For now, we assume that datasets fit into memory, but ideally we should have a path towards storing them on disk, especially if we add support for things like binarisation workflows and pre-processing later.

[ondrej33]

The working prototype was started by #29 and finished by #31. Few details remain, such as removing datasets and making all current events reversible.

[ondrej33]

Dataset removal and some other issues were fixed by #38. There is currently a bug in the dataset loading that we should address - when closing the dataset dialogue (instead of confirmation), it breaks the application state.

[ondrej33]

The dataset tab was largely refactored in PRs up to #53. The UI now supports more options for dataset editing, bugs were fixed, and the backend was refactored.