Fix blur behind folderview context menus

Summary:
Breeze has

if (!(widget->testAttribute(Qt::WA_WState_Created) ||
widget->internalWinId()))

KWindowSystem::blur(widget->winId(), ...)

which makes sense as the style should not be creating surfaces in polish
and I don't want to remove it.

This is problematic with QMenu::polish which has a subtle behavioural
change compared to QMenu::exec in that it calls ensurePolish before
creating the surface.

Plasma::ContainmentInterface accidentally has this workaround already
for parent window setting.

BUG: 395262

Test Plan:
Right clicked on desktop icon
Looked amazing

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: hein, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14174