slint-viewer: handle path being renamed and replaced

Some editors may backup a file by renaming it to append ~, then saving
the new file contents to a new file. notify-rs/notify#166 explains
that a non-recursive watcher on the parent directory is the
recommended way to deal with this situation.

Closes: slint-ui#3641

Author: 0x6e

tools/viewer/main.rs

@@ -190,6 +190,16 @@ fn init_compiler(
         .unwrap_or_else(|err| {
             eprintln!("Warning: error while watching {}: {:?}", args.path.display(), err)
         });
+        // Watch the parent directory incase args.path is renamed or removed
+        let parent = args.path.parent().unwrap();
+        notify::Watcher::watch(
+            &mut *watcher.lock().unwrap(),
+            &parent,
+            notify::RecursiveMode::NonRecursive,
+        )
+        .unwrap_or_else(|err| {
+            eprintln!("Warning: error while watching {}: {:?}", parent.display(), err)
+        });
         if let Some(data_path) = &args.load_data {
             notify::Watcher::watch(
                 &mut *watcher.lock().unwrap(),