Add Load method to editor classes and update corresponding headers

src/app/editor/system/settings_editor.h

@@ -209,6 +209,7 @@ class SettingsEditor : public Editor {
  public:
   SettingsEditor() : Editor() { type_ = EditorType::kSettings; }
   void Initialize() override;
+  absl::Status Load() override;
   absl::Status Update() override;
   absl::Status Undo() override { return absl::UnimplementedError("Undo"); }
   absl::Status Redo() override { return absl::UnimplementedError("Redo"); }