If you activate the Present Windows effect and then the Desktop Grid,
you'll be able to see the close button from PW. The reason for that is
PW doesn't destroy the close button and DG doesn't filter it out.
This patch addesses this problem by syncing DesktopGridEffect::isRelevantWithPresentWindows
On X11, the close button is filtered by the isSpecialWindow check. On
Wayland, the close button is filtered by the acceptsFocus check.
The proposed solution is kinda hack-ish, but on the other hand, we have
to keep those two methods in sync anyway.
In addition to the close button, notifications won't be displayed too.