WIP: based on commit 15bce7f75f0dfab9863db5d4c7a51f5fc96fdb89 in Godot.

Seems to work at runtime as well!

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Consistent with `visual_script` module.

Renamed `MultiScriptOwner` to `Mixin` respectively.

See https://stackoverflow.com/questions/925609/mixins-vs-traits

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Nodes extended with `MixinScript` are not very useful unless they have ability to be edited just like any other script from other languages.

This introduces `MixinScriptEditor` with the purpose of handling script editing. The editor will delegate script editing to other editors that handle `main_script`, if it's valid.

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Allows to conveniently attach main script to `MixinScript`. The complexity here is that we cannot use regular mechanism of attaching scripts, because usually scripts are attached to nodes, not scripts!

Allows to manually switch to mixin script editor when main script is edited automatically.

Those are not useful anymore, and they are limited to the number of letters. There's `MixinScriptEditor` which allows to edit `MixinScript` now.

Makes everything more consistent.

Unfortunately, we cannot provide overrides for arbitrary instances.
However, the real object is still available to all mixins via `owner`
property.

They are identical, and the former is removed in Godot 3.x.

Since there's no way to make arbitrary instances to use base types other than `Mixin`, there's not much benefit in disntinguishing between the "main" script and "mixins".

However, the first script added to `MixinScript` will be treated as the "main" one, in the sense that script editing is going to be automatically delegated to the first script.

This greatly simplifies both the core and the editor.