Refactor ROM handling and remove SharedRom singleton for improved architecture

- Eliminated the SharedRom class to enhance modularity and reduce global state management.
- Updated various classes to directly manage ROM instances, improving clarity and encapsulation.
- Added new functions for loading messages and colors from ROM, enhancing functionality.
- Refactored Canvas and Editor classes to utilize direct ROM references, streamlining interactions.
- Improved documentation and comments for better code understanding and maintainability.

src/app/editor/overworld/tile16_editor.cc

@@ -245,7 +245,7 @@ absl::Status Tile16Editor::DrawToCurrentTile16(ImVec2 click_position) {
 }
 
 absl::Status Tile16Editor::UpdateTile16Edit() {
-  auto ow_main_pal_group = rom()->palette_group().overworld_main;
+  static const auto ow_main_pal_group = rom()->palette_group().overworld_main;
 
   // Create a more organized layout with tabs
   if (BeginTabBar("Tile16EditorTabs")) {