This PR adds an option to only show the onion skins of the active object. The option is added to the Grease Pencil overlay menu in the 3D viewport. When enabled, only the onion skins of the active object are shown. This keeps the viewport a lot cleaner when working with multiple Grease Pencil objects. This resolves #136820. Pull Request: https://projects.blender.org/blender/blender/pulls/137405