add "duplicate" functionality in context menu for datasets

[okworx]

This would be an easy way for allowing the user to create a sort of template dataset which contains all of the information that is usually common to all datasets created in an organisation or of one batch.

This functionality could be added to the context menu for datasets.

Upon duplication of a dataset, only the UUID of the duplicate would need to be newly generated.

[msrocka]

This is done now for the context menu but it would be probably also good to bind this function to the Save as button in the toolbar of the respective editors.