remove unnecessary import.
add new abstract AlertButton.
Make NewVersionAlertButton a subclass of AlertButton;
Make MissingImagesAlertButton a subclass of AlertButton;
AlertPanel references AlertButton instead of NewVersionAlertButton directly.
Streamline code by using getAlertCaption() instead of isAlertTriggered();
added javadoc.
AlertPanel references AlertButton instead of MissingImagesAlertButton directly.
Update AlertPanel so it is simple to add a new AlertButton in future if required with a single new line in setAlertButtons().
simplify AlertButton subclasses by moving alertAction reference into AlertButton.
add javadoc to AlertButton.
don't display alert again until next restart if open download page.