Godot-proposals: Allow multiple CSG meshes to be merged into one

[golddotasksquestions]

Describe the project you are working on:
Prototype for level design.

Describe the problem or limitation you are having in your project:
The scene tree becomes increasingly chaotic the more CSG meshes are in the scene. Besides being chaotic, performance get's worse the bigger the tree and the number of Booolean operations. There is even an explicit warning about it in the docs.
Overview in the viewport becomes worse and worse with every CSG mesh. Even as few as 3 CSG boxes make it hard to tell the wireframe of the mesh apart from all the bounding boxes!
Boolean operations are usually an _intermediate_ step to get the part to a reusable unit. Not being able to merge CSG meshes together makes reuse ability hard and prototyping harder to manage.

This is just 3 boxes. Can't make a level with 3 boxes.

"Make the sure the objects children are not selectable" is helping with visibility, and is great for when you want CSGs to stay open and editable, but not with performance and I still would like to have to option to commit to this shape by merging it together, freeing up all overhead that a huge open CSG mesh tree would undoubtedly entail.

Describe how this feature / enhancement will help you overcome this problem or limitation:
The true power and potential of CSG meshes unfolds when infinite reusability, recombination and remerging of CSG meshes is possible. For this to happen we need the ability to

1. merge any CSG Meshes and CSG Primitives and
2. change MeshesInstances into custom CSG meshes and vice versa with a simple mouse click.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:
Viewport RMB click -> "merge CSG mesh and it's children",
Scene panel RMB click -> "merge CSG mesh and it's children",
Menu bar: RMB click -> "merge CSG mesh and it's children" ->

Describe implementation detail for your proposal (in code), if possible:
I can't pronounce "C" with classes.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
It would be used often by Level designers, people who quickly want to block out prototypes, gamejams games that require just very basic geometry. It would also integrate brilliantly with the upcoming Gltf2 export https://github.com/godotengine/godot/issues/14752

Is there a reason why this should be core and not an add-on in the asset library?:
It would be a imho necessary improvement to an existing feature to make is more usable for production work.

[fire]

I need to work on this, but this is usable for meshes, csg and gridmaps. https://github.com/godotengine/godot/pull/32218

Future work is animated characters and materials.

[Jummit]

Does this have to be limited to CSG, or is this a tool used for all 3d meshes? It could be put in the "Mesh" dropdown.
EDIT: Just read @fire s comment and I think he has the same point.

[jabcross]

Note: by code, you can call get_meshes() on the root object to generate an ArrayMesh, which you can put in a separate MeshInstance later. Adding a button in the resource's dropdown would be pretty easy.

[SuperDIMMaX]

CSG not merge, this is limitation, but you can merge as single mesh get_meshes() or exist addon _CSGExport_ (generate obj file).
But probality more usefull use blender or another editor for this, Godot now make non optimal CSG (generate a lot of vertex), this good only if you have procedual mesh, if you have this - get_meshes(), make mesh instance and del CSG. CSG needed if you morph mesh as boolean operation (hole in the any type wall) - i examle use for breaking glass