Add sound support for Linux.

Now UNTZ uses RtAudio in both Linux and Windows for audio.
Moved some files that are used on both Linux and Android to a common location.
Better code sharing between platforms in UNTZ.

Use WaveFileAudioSource on all platforms for RIFF wave files to have
equivalent functionality (and bugs).

Also refactored Sound::create() to avoid code duplication.
Fix Android build.