a TreeModel that's sortable and filterable?

[Joaquim Schmidlap <schmidlap schmidlap org>]

It looks like TreeModelFilter and TreeModelSort are separate  
derivations from TreeModel...pity.
If I wanted a TreeModel to be sortable and filterable, what's the  
best way forward? I assume I'd have to pick one and add the  
functionality of the other myself, or perhaps do something with  
multiple inheritance. What is recommended here? What's easiest?
And, I realize this calls for speculation, but can anyone say why  
this design decision was made? Seems like what I want to do would be  
relatively common.