Add play-waveform and play-sine to openal.examples
rosetta-code.metronome: use openAL to play sound

This brings cross-platform compatibility
rosetta-code.metronome: remove useless close-window call
rosetta-code.metronome: simplify stack shuffling
rosetta-code.metronome: correct disposal of timer
rosetta-code.metronome: add example as MAIN: