Don't pre-check the internal enabled flag when toggling decks.
Fixes lp:1138316
Add some documentation to SoundManager methods about concurrency issues.
Get rid of QHash for storing input buffers to delete and use a QList instead.
Remove request buffer mutex since there is only one clock reference device.
Move output buffer composition from SoundManager to SoundDevice.

This fixes a race condition in the use of the mono-downmix buffer. Multiple
soundcards could concurrently use the downmix buffer in SoundManager which leads
to data corruption. Making the downmix buffer owned by the SoundDevice prevents
this.
Remove accidental commit.
Adjust thread priorities according to discussion in Bug #1270583.
Show signal quality widgets if deck is configured, not just if it's enabled.
Reset user offset and sync adjustment when bpm or beatgrid change.