AppletPopup window type should be shown on all desktops.

AppletPopup's are typically supposed to be attached to the panel and
should remain visible when the virtual desktop is changed. If the
AppletPopup is pinned and the virtual desktop is changed, without
this patch, the window does not remain attached to the panel. This is
how AppletPopup was supposed to work even according to the unit test
which currently fails.

src/xdgshellwindow.cpp

@@ -489,6 +489,7 @@ void XdgSurfaceWindow::installPlasmaShellSurface(PlasmaShellSurfaceInterface *sh
         case NET::Notification:
         case NET::CriticalNotification:
         case NET::Tooltip:
+        case NET::AppletPopup:
             setOnAllDesktops(true);
 #if KWIN_BUILD_ACTIVITIES
             setOnAllActivities(true);