add clear_changes

src/model.rs

@@ -69,4 +69,15 @@ impl Taggart {
         self.changes = changes;
         self.changes
     }
+    /// Clear all modified tags.
+    pub(crate) fn clear_changes (&mut self) {
+        for entry in self.entries.iter_mut() {
+            if let Metadata::Music {
+                modified_tag, ..
+            } = &mut *entry.info.write().unwrap() {
+                *modified_tag = None;
+            }
+        }
+        self.changes = 0;
+    }
 }