Add features and start writing interfaces

engine/src/audio_engine/mod.rs

@@ -0,0 +1,12 @@
+mod jack;
+mod coreaudio;
+
+trait AudioBackend {
+    // Start a audio backend client
+    // It should be executed either on the startup,
+    // or when the audio backend is switched
+    fn start_client();
+    // Initialization of the client should happen 
+    // when a project is opened.
+    fn init_client();
+}