Start major sheet refactoring

src/main.rs

@@ -1,5 +1,6 @@
 mod database;
 mod sheet;
+mod sheet_dao;
 mod ui;
 
 use std::{
@@ -12,7 +13,7 @@ use database::Database;
 use env_logger::Env;
 use log::{debug, error};
 use relm4::RelmApp;
-use sheet::{OrphanFile, Sheet};
+use sheet::Sheet;
 use walkdir::WalkDir;
 
 use crate::ui::app::AppModel;