Because we currently don't have a way to detect when the popup is closed
we workaround the problem by never show the combo box as open.
Workaround #456
The button bevel was rendered upside down, which happens due to some
weird double y-axis inversion, which goes away when passing the QWidget
pointer to the style. When using the Qt backend, we can do that.
Set QStyle::State_Active conditional to whether the window is active,
which maps to Qt's active window concept and winit's has-focus (until we
render popups differently).
