Prepare for moving the AudioOutputSource to class AudioOutputControl.

That function is "noexcept".

release v0.20.9

This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

Add ffmpeg decoder support for *.adx files (Sega game console)

Closes #60

Patch from
 https://svnweb.freebsd.org/ports/head/audio/musicpd/files/patch-src_system_ByteOrder.hxx?revision=441921&view=co
(with a tiny modification)

Closes #59

Don't access AudioOutput::pause.

Don't access AudioOutput::pause.

This allows removing the "base.pause" check from the JACK output
plugin, and will allow removing similar accesses from other plugins.