Manual audio config controls

This series introduces sample rate and latency controls in the audio configuration dialog.  They can be used to find the optimal balance between low-latency and audio glitches.

When latency is too low it becomes harder for the CPU to keep up, resulting in audio glitches when audio data is not processed on time.  When latency is too high there is a noticable delay between playing a note and hearing it.

Unfortunately it is not possible to autoselect the right settings on every system so these manual controls allow the user to find the optimal values.