qt: call `QWidget::ensurePolished()` before setting transient parent in VLCMenu

`QVLCDialog::setWindowTransientParent()` first creates a window if it is not
already created.

Depending on the platform, a `QMenu` might use special attributes for its
underlying window. Some of these special attributes, such as, translucent
background requires them to be set prior to creation.

`QMenu` does such adjustments in polishing stage. Calling `ensurePolished()`
before calling `QVLCDialog::setWindowTransientParent()` satisfies that the
window is going to be what `QMenu` really wants.