SamplePlayer: Support simultaneous playback of multiple samples

[birdofpreyru]

Not a big priority for me right now, but it will be nice to support.

• It will require to modify SamplePlayer API, its .play() and .stop() methods, in particular, — they should return and accept a playback ID (similar to Android's SoundPool API), rather than sample names. This will permit to play multiple instances of the same sample.
• On Android, it presumably just requires to correctly configure SoundPool object.
• On iOS it requires some updates in the implementation (essentially there we have to reproduce SoundPool object ourselves, using lower-level components of AVAudioEngine API).