Attempt to fix https://prototype.lighthouseapp.com/projects/8886/tickets/1282-latest-prototypejs-from-github-repo-fails-in-opera

Opera (at least 11.X) has false positive result in feature test `'styleFloat' in DIV.style`, but should have its own `getStyle_Opera` and standard `setOpacity`/`getOpacity`.

Another variant:

    if (Prototype.Browser.Opera) {
      // Opera also has 'styleFloat' in DIV.style
      methods.getStyle = getStyle_Opera;
    } else if ('styleFloat' in DIV.style) {
      methods.getStyle = getStyle_IE;
    }
    if (!STANDARD_CSS_OPACITY_SUPPORTED) {
      methods.setOpacity = setOpacity_IE;
      methods.getOpacity = getOpacity_IE;
    }