[tui] Set tab title to file name.

Also update to use anyhow::Result in some places.

src/gui.rs

@@ -1,10 +1,10 @@
+use anyhow::Result;
 use cxx_qt_lib::QString;
-use std::error::Error;
 
 pub mod colors;
 pub mod filesystem;
 
-pub fn run(root_path: std::path::PathBuf) -> Result<(), Box<dyn Error>> {
+pub fn run(root_path: std::path::PathBuf) -> Result<()> {
     println!("Starting the GUI editor at {:?}", root_path);
 
     use cxx_qt_lib::{QGuiApplication, QQmlApplicationEngine, QUrl};