Make completion more efficient

This change contains no "functional / behavioural" change to the completion stuff. Changes are as follows:

• Use std::vector for filtered and prefilter in the model.
• Remove dead functions
• No "prepends", just push_back and reverse the vector later
• No need to do upper_bound when we are adding items to the model initially. Just add everything and sort it afterwards.
• Minor optimizations to the matching / filtering code.