Draft: Refactor ImportDatasetWidget

Description

While implementing the importing of datasets from Kaggle.com, I thought that it will be good to decouple the ImportDatasetWidget from the implementation for the currently supported dataset collections. I moved the implementation for the currently supported dataset collections to PublicArchiveDatasetCollection. The plan is to have an abstract class DatasetCollection, which the PublicArchiveDatasetCollection, and the future KaggleDatasetCollection will inherit from. The ImportDatasetWidget will then use the public interface of the DatasetCollection abstract class in its implementation.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes:

When external dependencies are removed

• Reporting to ...