many doxygen @todo items should be tickets

[rossb]

The doxygen @todo feature has been used to document unresolved issues which should be listed as track tickets: http://www.portaudio.com/docs/v19-doxydocs/todo.html

Resolution involves resolving the issues or moving the items into trac tickets.

We should probably have a policy of not using @todo except for making short-term notes.