Add ability to drag nodes between scenes

[qarmin]

Godot version:
3.1 0e27af2

OS/device including version:
Windows 10

Issue description:
Now is impossible to drag a node between scenes but it will be easier to reuse some nodes without creating new scenes.

I think that the best option will be allow to drag node over another scene tab and then should Godot open this scene to allow to drop desired node.

Steps to reproduce:

[KoBeWi]

When you right-click a node, there's a "merge from scene" option (or something similar). While it's not as neat as ability to drag and drop between scenes, the functionality is already there.

[Flavelius]

This only works when the nodes are part of a saved scene, plus it has the downside of keeping the original file for sub resource location.
There are a variety of proposals for drag and drop support already, allowing cross-scene moving of nodes in the editor should be considered when tackling those.

[KoBeWi]

This will be probably solved by #34892

[KoBeWi]

Resolved by node copy-paste. Open a proposal if this is still desired.