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.