Custom texture referencing in scene

A scene can use user provided texture files e.g. for the sky texture. These files can either be in the scene directory or somewhere else in the file system (absolute path).
To make it easier to copy a scene to a different computer, it would be helpful to have these files in the scene directory.
On the downside, this could lead to duplicate files e.g. when using large skymaps.
I suggest to add UI indicators besides the custom file controls that show the status of the file (embedded/absolute) and a button to embed (copy into scene directory) the file.
The next step after that could be to create a symlink in the scene directory to the external file and only allow embedded scene files in the future.

Begin by locating the scene custom file controls and tracing how they represent files in the scene directory versus absolute paths. Review the handling of sky textures and other user-provided texture files, then define the UI behavior for showing embedded or absolute status and copying an external file into the scene directory. Done means the proposed workflow is specified and consistently supports those file states.

Written by the indexing model from the issue text.