Restore ofxOpenALSoundPlayer for iOS

This PR restores the ofxOpenALSoundPlayer and SoundEngine files for iOS. This is to provide a fallback sound player for the regression documented in issue #2336

The files are added to the iOS+OFLib project, but they aren't pulled in by any of the headers. So, you'd have to explicitly `#include "ofxOpenALSoundPlayer.h"` and also create it like `ofxOpenALSoundPlayer myPlayer`.

Tested by replacing the "beats" sound player in the soundPlayerExample with an explicit ofxOpenALSoundPlayer, and didn't notice any conflicts.

For the future (as in 0.8.1) I think it'd be a good idea to remove these again, but provide a solid "low level" sound player which sacrifices a bit of the automatic niceness of the AVFoundation player, but allows you to have tighter control over your playback. Initial testing with an AUFilePlayer-based sound player went very well, but it failed the

> works perfectly

test, due to not having a total 1:1 coverage of the features in the OpenAL player (like playback at different pitches). It did cover the use case of high-speed seeking within a file, though.

Pinging @ofTheo @julapy @edburton