Don't cache value of popupWindow property in EffectWindow

Summary:
With the plasma shell protocol, a ShellClient gets its role/window type
after it's created, so the cached value of the popupWindow property in
EffectWindow can be wrong.

In addition to that, the value of the popupWindow property still can be
wrong because plasma shell surfaces don't have xdg-popup role, so we need
to check NetWM-based window type in ShellClient::isPopupWindow.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16835