Expose EditorNode to plugins

[vnen]

The core editor plugins have major advantage over GDScript plugins: the access to the EditorNode singleton. Many of the EditorPlugin functions are just wrappers to EditorNode methods.

The core C++ plugins are quite good examples on how to extend the editor but they require unavailable API. For instance, the Theme editor is quite good for being context sensitive. You can't do that in a custom plugin since it's impossible to show and hide the bottom panel without access to EditorNode.

Is there a reason why EditorNode is not fully exposed?

The alternative, of course, is making more EditorNode wrappers inside EditorPlugin like I did in #6516.

[reduz]

EditorNode changes a lot and the overall interfaces are pretty dirty. The interfaces are abstracted in EditorPlugin just to avoid the mess of EditorNode and provide an API that will be unlikely to change

[vnen]

@reduz looking at it now I can understand, but WDYT of my last phrase:

The alternative, of course, is making more EditorNode wrappers inside EditorPlugin.

There are some useful things to use like reload_scene() for importers.

[kubecz3k]

I added 3.1 milestone since I remember reduz was talking about EditorNode refactoring in 3.1. So I guess we will consider then what to do with this issue.

[akien-mga]

Moving to the next milestone as 3.1 is now feature frozen.

[akien-mga]

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