Add PortAudio non-blocking stream support

[kareandersen]

• Supports non-blocking (callback based) streams
• Still supports blocking io
• Now we can suspend/resume properly on Linux in the text-mode player, restart the tune etc. without issues (tested to the best of my ability)
• WIP on tweaking the latency

[kareandersen]

Ok, let's see if we can tweak the latency for both non-blocking and blocking scenarios. It may not be possible to make a "perfect" solution for all systems, instead it may be an idea to let the user override the latency (perhaps even interactively in the text-mode player).

I just had a quick read through at the BufferingLatencyAndTimingImplementationGuidelines wiki page and found it to be a very well written and informative wall of text.

The current code is simply accepting the value provided for the default device's defaultLowOutputLatency, which may be ok - but I have not yet had a look at what the actual value provided is and if it is indeed based on buffer length or other factors. What I have noticed is that I can sometimes get glitches with the ALSA backend (possibly due to a configuration issue on the host machine) but not on PortAudio (which could also be explained by the relatively huge buffer I set up).

I'll poke around a bit and see what's up on multiple systems while digging into the documentation.

Edit: Let's keep this in the scope here too: Latency rationale documentation commit