Make it possible for QueryMakers to define which levels of filters they accept.
The default implementation is to accept everything. Some QueryMakers ( Such as Ampache and Mp3Tunes ) do not support filters for all the levels in the collection tree, and thus override the validFilterMask() function to return a mask of the filter levels that they support.

The collectionTreItemModelBase has been updated to honor this and only add filters that are accepted. The AmpacheQueryMaker now only accepts Artist filters.

This system will also be really useful for making the FilterEditor aware of which filter options should be available depending on which collection we are building filters for.