core: Extend Category import with an option to force deletion of categories in use
Category spreadsheet import did not delete categories if some documents where related to these categories. While this may make sense when using this import in site with already some data, in many cases, we want to be able to delete the categories, so that the result is that the category tree match the imported spreadsheet.
This extend the dialog to support a new "force delete" mode that delete categories even if they are use, leaving the documents using categories with broken relations.
