Refactor welcome screen behavior in EditorManager to improve user control

- Updated the logic for displaying the welcome screen to only show it if it hasn't been manually closed by the user.
- Introduced a new flag to track whether the welcome screen was manually closed, enhancing user experience by preventing unwanted re-displays.
- Reset the welcome screen state when a ROM is loaded, ensuring a fresh start for the user.
- Minor adjustments to comments for clarity and maintainability.

src/app/editor/editor_manager.h

@@ -164,6 +164,7 @@ class EditorManager {
   bool show_global_search_ = false;
   bool show_session_rename_dialog_ = false;
   bool show_welcome_screen_ = false;
+  bool welcome_screen_manually_closed_ = false;
   size_t session_to_rename_ = 0;
   char session_rename_buffer_[256] = {};