More robust midi implementation

src/settings.rs

@@ -6,8 +6,10 @@ const APP_NAME: &str = "cagire";
 
 #[derive(Debug, Default, Serialize, Deserialize)]
 pub struct MidiSettings {
-    pub output_device: Option<String>,
-    pub input_device: Option<String>,
+    #[serde(default)]
+    pub output_devices: Vec<String>,
+    #[serde(default)]
+    pub input_devices: Vec<String>,
 }
 
 #[derive(Debug, Default, Serialize, Deserialize)]