Long-term "TODO"
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- output plugins should have some kind of (optional) "query available formats"
  function, so that the core could determine what should be used. perhaps
  offer users this info?

- maybe all audio plugins (input, output, effect) could be made completely
  re-entrant. this probably means severely breaking the API. not very
  trivial either .. some plugins are probably impossible.
  * basically plugin init() should allocate / init a state struct to
    be used in all operations, etc.
  * context support should be added to plugin functions.
  * question: issues with plugins that use non-re-entrant library backends
    etc?

- URIs with "?" are stripped (the subtune support uses that), which is bad,
  because some (a lot) of real-world URLs use 'GET' arguments. this breaks
  probing, but current input probing is confusing (and possibly broken anyway)
  and should be refactored.
  * As of 2.5, is this still a problem?

- mime-types support:
  * there is already code for mime support, but I think it may not be
    sufficient as it is designed for input plugins only --
    also playlist containers etc. need this (IMHO)

- document the different APIs via Doxygen

- plugin rewrites:
  * modplug (in progress by ccr)
