Conversion to `<generic>` controllers and robot windows for remote PROTOs imported locally

[ygoumaz]

As discussed in #4898, remote PROTOs that are imported locally when editing them in Webots (right-click on PROTO + Edit PROTO Source + Copy/Import message) are not coming with their respective controllers and robot windows, which means that, after save and reload, they are not found as they were not copied to the local project.

Currently, relocating a project usually copies the controllers inside the new folder. Also, #4898 updated the PROTO wizard to generate <generic> fields for windows and controllers from template PROTOs.

Therefore, it would be more consistent and user-friendly to also update these fields to <generic> when importing remote PROTOs to the local project.