Don't create() windows that aren't native, upon receiving a palette change event.

Test Plan:
run kmail or falkon, then run kcmshell5 fonts, change font size

ASSERT: "!d->offscreenWindow->handle()" in file /d/qt/5/kde/qtdeclarative/src/quickwidgets/qquickwidget.cpp, line 1014

Apparently that QQuickWindow should not be create()d so calling winId()
unconditionally here is a bad idea, skip non-native windows.

Reviewers: carewolf, davidedmundson, broulik

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23808