Move Sheet and Composer to separate code file

src/main.rs

@@ -1,8 +1,9 @@
 mod database;
 mod mcdu;
+mod sheet;
 mod sheet_listing;
 
-use std::{env, path::PathBuf, process};
+use std::{path::PathBuf, process};
 
 use clap::Parser;
 use database::Database;
@@ -13,8 +14,6 @@ use mcdu::McduModel;
 use relm4::prelude::*;
 use sheet_listing::{SheetListingInput, SheetListingModel};
 
-use crate::database::Sheet;
-
 struct AppModel {
     mcdu: Controller<McduModel>,
     sheet_listing: Controller<SheetListingModel>,