Upgrade from SoundTouch 1.6.0 to 1.8.0.

Fixes a portability-related segfault on Windows x64 reported in
Bug #1402219.
Call SoundSource::setChannels in SoundSourceOggVorbis and SoundSourceSndFile. Fixes Bug #1402262.
Add unit test that we can open AIFF, FLAC, MP3, Ogg, and WAV files using
the pre-existing cover-art test files.