Improve output socket handling of the Value Changed node

[MoritzBrueckner]

Previously the "Unchanged" output of the Value Changed node was not activated if the value was unchanged compared to the last value, but only if it was different to the initial value from the first execution of the node. The "Changed" output however compared the last value.

This led to some confusion, so there is now a new output "Is Initial" for the old functionality, and "Unchanged" now checks against the last value as well. It's also possible now to use null as a value without always triggering the initialization of the node.