feat(editor): enhance ScreenEditor with layer visibility controls and custom map loading/saving

- Added checkboxes for toggling visibility of title screen background layers (BG1 and BG2).
- Implemented a new method for rendering the composite view of the title screen.
- Introduced functionality for loading and saving custom maps from external binary files, enhancing user flexibility in map management.

Benefits:
- Improves user experience by allowing dynamic control over layer visibility during title screen editing.
- Expands the capabilities of the ScreenEditor with custom map handling, facilitating easier map modifications and sharing.

src/app/editor/graphics/screen_editor.h

@@ -67,6 +67,7 @@ class ScreenEditor : public Editor {
   void DrawInventoryToolset();
 
   // Title screen layer editing
+  void DrawTitleScreenCompositeCanvas();
   void DrawTitleScreenBG1Canvas();
   void DrawTitleScreenBG2Canvas();
   void DrawTitleScreenBlocksetSelector();
@@ -134,6 +135,8 @@ class ScreenEditor : public Editor {
   bool title_h_flip_ = false;
   bool title_v_flip_ = false;
   int title_palette_ = 0;
+  bool show_title_bg1_ = true;
+  bool show_title_bg2_ = true;
 
   // Overworld map screen state
   int selected_ow_tile_ = 0;