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.
Extract the logic of importing the content of a graph within a graph instance from
the graph loading logic.
Add `Graph.importGraphContent` and `Graph.importGraphContentFromFile`
methods.
Use the deserialization API to load the content in another temporary graph instance,
to handle the renaming of nodes using the Graph API, rather than manipulating
entries in a raw dictionnary.
New test suite that focuses on testing the behavior of the attribute
changes callback system when such an attribute is connected to
an upstream dynamic output value.
