[klipper] Call ensurePolished() before native window of menu gets created

It's important to have ensurePolished() being called before the native
XCB window gets created. The widget style (in our case Oxygen) sets the
RGBA hint during that call. Once the native window is created this would
fail.

In the case of KlipperPopup the native window gets created by accessing
the winId(), thus the ensurePolished() needs to happen before.

REVIEW: 118199