Allow for multiple listeners in DefaultTimeBar.java

[hallyhaa]

Issue description

DefaultTimeBar has a setListener method, but no addListener. It is impossible to add listeners without overwriting the default listener behaviour. It would be very nice to be able to just add one more listener to have one's own logic executed in addition to the default.

I believe the only changes needed in the code, are something like

    private List<OnScrubListener> listeners = new ArrayList<OnScrubListener>();

    public boolean addListener(OnScrubListener listener) {
        return listeners.add(listener);
    }

    public boolean removeListener(OnScrubListener listener) {
        return listeners.remove(listener);
    }

... and then on each use of the listener, where today the code simply checks for null, one could replace the if-null-check with a for-loop, thus (example taken from onTouchEvent():

  for (OnScrubListener listener : listeners) {
    listener.onScrubMove(this, scrubPosition);
  }