TrigOutputHandling: use OrderedSet semantics in TriggerEDMSerialiserTool

Use `OrderedSet` semantics for the "CollectionsToSerialize" property
instead of defining our own merge semantics in Python. Note that we
could have also just used a `set::set` in C++, which would have the same
effect (but the Python type would still be `list`, which is a bit
confusing maybe).