define midi ins and audio outs in project

src/model.rs

@@ -56,10 +56,10 @@ pub struct App {
     /// Paths to user directories
     xdg:               Option<Arc<XdgApp>>,
     /// Main audio outputs.
-    audio_outs:        Vec<Arc<Port<Unowned>>>,
+    pub audio_outs:    Vec<Arc<Port<Unowned>>>,
     /// Number of frames requested by process callback
     chunk_size:        usize,
-
+    /// Transport model and view.
     pub transport: TransportToolbar,
 }