Allow editable children to be on by default

[ghost]

Godot version:

3.1

Issue description:

There are some cases where I always want a scene to have its children editable, as they are meant to be changed once the scene is instanced; needing to enable this option one-by-one is tedious. The following suggestions can help facilitate this:

1. Add an editor option to enable editable children by default; this results in one click to collapse editable children, rather than two to show them.
2. Add an option for PackedScenes when right-clicking them to set whether editable children should be on by default.
3. Add an option for nodes when right-clicking them to set whether they should always be editable for an instanced scene, regardless of whether editable children is enabled (e.g. Always Editable.)

[Jummit]

4. Make them not editable by default, but have a little button to easily enable / disable the feature.

[ghost]

@Jummit that's a repeat of suggestion 1, unless you mean keep the existing situation and don't offer any further configuration options, which wouldn't be of any benefit unless you just happen to hate having options. As I said, it's tedious to enable this option on each instanced scene one-by-one, when you always want it to be enabled from the start; suggesting there should be no change would be like saying there should be no way to configure the import process and you should just set those flags every time you import something, even if you want certain flags to always be enabled.

[frojo]

And/or have a toggle in the Project Settings for "Scene instance children are editable by default."

[volzhs]

I guess it depends on development style.
I mostly do not use editable children.

[BraindeadBZH]

This feature is especially useful when developing a plugin which can modify the scene tree. At the moment it is not possible to save the changes made to a scene node instantiated and modified through a tool script, because you cannot toggle "Editable Children" through script.

[hilfazer]

5. Add a context menu option: Instance Child Scene (Editable)

[Calinou]

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