title: Qt::WA_TranslucentBackground available everywhere?
body: <p>I have a custom widget to emulate balloon tips. To be displayed properly, the widget depends on the QWidget attribute <code>Qt::WA_TranslucentBackground</code>. My application should work on all major platforms (Windows XP, Windows 7, Linux, Mac), so I worry a bit: Is this attribute available on all major platforms? If not, can I query if it is? <code>testAttribute()</code> doesn't do that. It only returns whether the attribute has been set, not whether setting it has an effect.</p>