Improve 3D preview visibility in the advanced import settings dialog

[Bartusew]

Describe the project you are working on

3D game

Describe the problem or limitation you are having in your project

There are three things I'd like to address here.

1. Enviroment - When you open scene import settings (or what other can call "Advanced import settings for 3D scene")
   a window with a imported scene is showing up, but with very plain gray enviroment.
   And as a default enviroment this can be ok, but in some cases it does not help with visibility of the imported scene.

[picture shows some gears from the bottom side, almost everything blends with the background]

2. External materials - When you select a material and enable use external material
   and select a path to different "in project" material.
   Nothing changes in scene import settings.
   Which is inconsistent in my opinion, user (at least like me) would expect to show the selected material and other made changes at runtime.

Example:
[the old material is still used]

[while the "external material" specified in the setting looks like this]

3. Different draw modes - now in scene import settings there is only just normal 3D showcase of imported thing.
   If my explanation is still not clear enough then for example in normal 3D editor
   we have an option to switch to "wireframe" or "unshaded"
   which would be nice to have in scene import settings.
   

Describe the feature / enhancement and how it helps to overcome the problem or limitation

1. Enviroment - I think convenient would be to make a new setting inside project settings "editor/import/" and for example something like "scene_import_enviroment" were user can select a path to the already existing enviroment inside of project.
   And this enviroment will be loaded every time when a user wants to edit the imported scene, (could also be loaded inside class constructor, but it will require user to reload the entire project to see change) This will help in some cases where geometry blends with the background.

2. External materials - when user selects external material from project the material preview, and meshes will switch to that external material. Just like some of people would expect. This way if user want to change something in import settings the preview will update accordingly which will help a lot, since now we only can se changes in actuall scenes but not in the settings window..

3. Different draw modes - in the top left corner where "Actions..." button is located the new button would be made, and by clicking it it would allow changing to different render modes like mentioned before "wireframe", "unshaded" etc.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

1. Enviroment
   By changing path in "scene import enviroment":
   
   the enviroment should change accordingly:
   [now we can clearly see the bottom of the gears]
   

2. External materials
   By changing option inside material to use external material inside project the material preview will be updated
   [Material preview]
   
   [mesh preview]
   

3. Different draw modes
   Additional button to change render mode
   
   [scene preview with wireframe render mode selected as a showcase]
   

If this enhancement will not be used often, can it be worked around with a few lines of script?

It may be done using GDscript, or even GDExtension, but this feature would be better to be implemented into the engine.
(at least in my opinion)

Is there a reason why this should be core and not an add-on in the asset library?

like I said "for convience". If you people think differently I won't complain.

[Calinou]

Enviroment - When you open scene import settings (or what other can call "Advanced import settings for 3D scene")
a window with a imported scene is showing up, but with very plain gray enviroment.
And as a default enviroment this can be ok, but in some cases it does not help with visibility of the imported scene.

This is being addressed by godotengine/godot#75787.

External materials - When you select a material and enable use external material
and select a path to different "in project" material.
Nothing changes in scene import settings.

This is being tracked in #7238, which is implemented by godotengine/godot#86430.