Added ScriptableObject version of the ToDo list. Works identical to the MonoBehaviour version, just isn't attached to a Scene.
Added ToDo lists to Create menus and "Cyan" heading to AddComponent menu
Added SceneReferencesHandler which is automatically added to a scene when required.
It's now possible to link objects to a task which are cross-scene, or scene-based on a prefab or ScriptableObject ToDo list. This has only been tested in 2020.1 currently.

How it works :
When dragging an object onto a task from a different scene, or from a scene when the ToDo list is a prefab/ScriptableObject, a SceneReferencesHandler object is created and added to the scene - hidden from the Hierarchy but still serialised in scene. The dragged object is registered with the handler, which assigns it a unique ID, adds it to a SerializableDictionary and saves the scene. The task can then reference and serialise the SceneAsset (in element.objectReference) and the ID (in element.objectReferenceID). When the task is visible in-editor, it will attempt to obtain the SceneReferenceHandler from the SceneAsset (if loaded, sidenote : won't work at runtime sadly, would need to save the scene name/path/buildIndex instead). With this it can obtain the object from the ID and show it in the object field. Fields which are serialised in this way are marked with a asterisk.

Fixed issue with reference handler not being marked dirty, so wasn't being saved. Thought Undo.RecordObject would handle it, but apparently it doesn't when using HideFlags to hide from hierarchy.

SceneReferenceHandler is now correctly serialised in 2018/2019 versions.

Fixed issues with saving prefab instance modifications
Fixed issue with dark mode text colour. Text cursor also now uses the same colour as the text.
Removed debug messages

GameObject Create menu now shows as "To Do List (MonoBehaviour)"
Asset Create menu now shows as "To Do List (ScriptableObject)"

Added UNITY_2019_2_OR_NEWER check for TryGetComponent usage in GetSceneReferencesHandler to prevent error in previous versions. Just uses GetComponent and null check instead.