Categories need to use reference to filters

[irockel]

Currently, categories reference the assigned filters by name. If the name of a
filter is changed, the category no longer is able to locate the filter. The
correct relationship is to have the category hold a reference to the filter object.

Deleting a filter currently does not check to determine if a filter is currently
being referenced by a category. Filters that are referenced by a category should
not be deleted.

Merged from here: https://java.net/jira/browse/TDA-28