Don't include embedded contrainers in containmentAt(pos)

Do to so means the System Tray applet ends up being placed inside the
system tray container, which ends up in a parent loop, causing a stack
overflow every time parent() is called recursively

This happens whenever one drags a sys tray on the desktop or simply adds
a new system tray whilst running.

BUG: 361777

We also need to do a hack for Plasma/5.7 in plasma-desktop somehow in
all cases where this is used.

src/scriptengines/qml/plasmoid/containmentinterface.cpp

@@ -277,6 +277,9 @@ QObject *ContainmentInterface::containmentAt(int x, int y)
         if (contInterface && contInterface->isVisible()) {
             QWindow *w = contInterface->window();
             if (w && w->geometry().contains(QPoint(window()->x(), window()->y()) + QPoint(x, y))) {
+                if (c->containmentType() == Plasma::Types::CustomEmbeddedContainment) {
+                    continue;
+                }
                 if (c->containmentType() == Plasma::Types::DesktopContainment) {
                     desktop = contInterface;
                 } else {