Add Load method to editor classes and update corresponding headers

src/app/editor/dungeon/dungeon_editor.cc

@@ -41,6 +41,10 @@ constexpr ImGuiTableFlags kDungeonObjectTableFlags =
     ImGuiTableFlags_Hideable | ImGuiTableFlags_BordersOuter |
     ImGuiTableFlags_BordersV;
 
+absl::Status DungeonEditor::Load() {
+  return absl::OkStatus();
+}
+
 absl::Status DungeonEditor::Update() {
   if (!is_loaded_ && rom()->is_loaded()) {
     Initialize();

src/app/editor/dungeon/dungeon_editor.h

@@ -43,6 +43,7 @@ class DungeonEditor : public Editor, public SharedRom {
   DungeonEditor() { type_ = EditorType::kDungeon; }
 
   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"); }