Allow specific classification values to be exluded from classification browser

In some instances, it is desirable to exclude some values from the classification browse.

An example: A list of publications that need to be processed by editors, ordered by state.

• Adding, say restriction="-state:(published blocked deleted)" is not an option, because the unnecessary values are still displayed (with numerical value 0).

• Alleviating this with emptyleaves="true" is also not an option, because this removes wanted values (when there happens to be no object in the given state).