Copy DataEngine relays before itterating

relay->checkQueueing emits a signal to all observers. As it's a signal
anything can happen at that point. It's seemingly common for a receiver
to connect/disconnect a new visualisation which in turn alters
d->relays.

This broke when a foreach (with implicit shallow copy) was ported to a
for statement.

This report contains a good analysis and test case

BUG: 422973

Probably also:

BUG: 421392
BUG: 423713
BUG: 423776
BUG: 422230
BUG: 423081

src/plasma/datacontainer.cpp

@@ -353,7 +353,9 @@ void DataContainer::checkForUpdate()
     if (d->dirty) {
         emit dataUpdated(objectName(), d->data);
 
-        for (SignalRelay *relay : qAsConst(d->relays)) {
+        //copy as checkQueueing can result in deletion of the relay
+        const auto relays = d->relays;
+        for (SignalRelay *relay : relays) {
             relay->checkQueueing();
         }