wayland: Return a type of NET::Unknown for popups

This is to ensure that isNormalWindow() returns false for popups. One
could argue that we abuse netwm window types, on the other hand, we
don't know the exact type of popups and NET::Unknown is the closest type.

src/xdgshellclient.cpp

@@ -1777,7 +1777,7 @@ NET::WindowType XdgPopupClient::windowType(bool direct, int supported_types) con
 {
     Q_UNUSED(direct)
     Q_UNUSED(supported_types)
-    return NET::Normal;
+    return NET::Unknown;
 }
 
 bool XdgPopupClient::hasPopupGrab() const