Save/restore selection history to/from disk when closing and reopening Unity

[acoppes]

Now that we are using global identifiers, we could try storing all the history serialized in a file or editor user preference when the editor is quitting or when changing from scene to scene, and then restoring it when the editor is opening or when the selection history is initialized.

[acoppes]

Just some references:

• https://docs.unity3d.com/ScriptReference/SceneManagement.EditorSceneManager.SceneClosingCallback.html
• https://docs.unity3d.com/ScriptReference/EditorApplication-quitting.html
• https://forum.unity.com/threads/whats-the-usersettings-directory.754436/