Add Load method to editor classes and update corresponding headers

src/app/editor/sprite/sprite_editor.cc

@@ -23,6 +23,10 @@ using ImGui::Text;
 
 void SpriteEditor::Initialize() {}
 
+absl::Status SpriteEditor::Load() {
+  return absl::OkStatus();
+}
+
 absl::Status SpriteEditor::Update() {
   if (rom()->is_loaded() && !sheets_loaded_) {
     // Load the values for current_sheets_ array

src/app/editor/sprite/sprite_editor.h

@@ -38,6 +38,7 @@ class SpriteEditor : public SharedRom, public Editor {
   SpriteEditor() { type_ = EditorType::kSprite; }
 
   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"); }