Finer Clear methods for queries

[LPeter1997]

Right now clear just clears non-input queries after a given revision. We should be able to have a finer-grained system. We should:

• allow clearing input queries (and there the changed version should matter, not verification number is meaningless)
• allow to filter for certain queries only when clearing

Maybe an API like the below in IQuerySystem should be enough.

public void ClearWhere(/* some predicate type */);