Tooltips of Properties editor are out of place

1. Open a qml file in Design mode.
2. Select an item.
3. In Properties editor, hover a label.
   An explaining tooltip will appear in a completely wrong place. Other tooltips (not from Properties editor) look fine.

Tooltips should be displayed next to the control which they explain.

Note: We changed back to QQuickWindow, so the bug in not visible in Qt Creator 3.2. But we will change to QQuickWidget in master after the release of Qt Creator 3.2.