Appending Nodes Instead of Whole Tree

[Vollkornaffe]

An idea from @BradyAJohnston is to have the import append nodes to the current tree rather than create a new one.

There is a tricky component to this: the tree's interface.
We have to interact with it as soon as there are group input or group output nodes in the serialization set.

For one, our current importing logic can't handle existing interface items. At the very least, the order and panel-parenting will be messed up.
This is something to reevaluate after #58, though.

But maybe more fundamental: What behavior are we going for?

Say, one pastes different parts of the same original tree; should it replicate the interface items x times?
A heuristic to match existing interface items could be more confusing than helpful.

Alternatively, we could drop input/output nodes, their links, and ignore the interface altogether.
That matches the behavior of un-grouping groups in Blender.

[Vollkornaffe]

Another idea from @kolibril13: Import the whole tree but have it in the current tree as a group node.

One could still un-group that and get this behavior:

Alternatively, we could drop input/output nodes, their links, and ignore the interface altogether.