Preventing form submission on button controls

1. Changed type attribute to "button" on <button> elements where it wasn't already set and thus used to fall back to type="submit", which leads to submitting the page on click if control is contained within a <form> element (e.g. in an ASP.NET or Oracle APEX application).
2. Moving event.preventDefault() calls to handleClick_. Calling preventDefault() inside a handler for mouseup is useless, since only click or keypress are responsible for triggering the DOMActivate event which the form is actually submitted on. Since we use type="button", most browsers already won't submit anyway, but this case probably needs some more testing in the older ones.