Yet another node copy-paste PR

Created by: KoBeWi

Following the failure of #31616, here's another attempt to close #3720 (counting all PRs so far, isn't it 5th one already?). I'll be angry if this one isn't merged eventually :/

In my previous PR, the problem was that copy-paste wasn't working like if you duplicated the node, so now I literally used duplicate(). The PR introduces node_clipboard variable, which is a list of copied nodes. Basically you can select multiple nodes and they will be duplicated into the list. If you copy, but the list already has elements, they are freed.

Paste makes yet another copy of these nodes and puts them under the node you have selected. The fact that I used duplicate (duplicate_from_editor() to be precise) ensures that all references and everything is correctly kept. If you copy a node with resource, the pasted node will still reference the same resource. What's more interesting, you can copy a scene root and paste it into another scene, which will create an instance (xd). Paste has already anti-cyclic-reference safeguards.

The Cut operation might be arguable though. When you cut-paste a file, the original file is moved to the new location. I tried to do that, but I've ran into infinite number of reference-related problems, so my Cut operation works like in text editors. It's the same as Copy, but also deletes the node.