midi: add pgup/pgdn; cleanup

crates/app/src/model.rs

@@ -182,6 +182,14 @@ impl Tek {
         }
     }
 
+    pub fn toggle_modal (&mut self, modal: Option<Modal>) {
+        self.modal = if self.modal == modal {
+            None
+        } else {
+            modal
+        }
+    }
+
     // Create new clip in pool when entering empty cell
     pub fn clip_auto_create (&mut self) {
         if let Some(ref pool) = self.pool