Changing attributes type when active filter on that attribute

how to reproduce:
- open a graph
- filter on an attribute for instance a category
- go to data and change column type to keywords
- filter is still in "category" mode

This situation is not broken. The filter acts just as if the column was categorial.

Options to do things differently:
- change filter type according to attribute new type: very difficult to do as we would need to adapt filter parameters which is actually impossible in many cases
- desactivate/remove the filter: destructive action which might not be understood by users
- just warn the user: we could warn the user before he/she changes teh column type that there is an active filter on it