Add two methods that are local to tests, `registerNodeDesc` and
`unregisterNodeDesc`, which handle the registration and unregistration
of `NodePlugins` from a node description. This reduces the amount of
code in tests whenever `NodePlugin` need to be instantiated prior
to their registration and so on.
The desc.Node.onCreated callback is a hook in the Node API for developers to
write custom behavior on a node first initialization.
By being called in the core.Node constructor, this was triggered in several situations
that don't match this idea and with unpredictable side effects (graph loading, node re-creation on undo...).
* Make `onNodeCreated` callback an explicit method on desc.Node.
* Remove the call to node descriptor's `onNodeCreated` callback outside core.Node constructor.
* Trigger this callback on explicit node creation (adding new node to graph, init a graph from a template).
