App architecture modifications.

Now GUI and the app itself is divided into two different binaries.
The app also provides system tray icon.
Whereas the GUI can be used to configure the app.

gui/src-tauri/src/tauri_commands/audio.rs

@@ -0,0 +1,33 @@
+use pv_recorder::RecorderBuilder;
+
+#[tauri::command]
+pub fn pv_get_audio_devices() -> Vec<String> {
+    let audio_devices = RecorderBuilder::default().get_audio_devices();
+    match audio_devices {
+        Ok(audio_devices) => audio_devices,
+        Err(err) => panic!("Failed to get audio devices: {}", err),
+    }
+}
+
+#[tauri::command]
+pub fn pv_get_audio_device_name(idx: i32) -> String {
+    let audio_devices = RecorderBuilder::default().get_audio_devices();
+    let mut first_device: String = String::new();
+    match audio_devices {
+        Ok(audio_devices) => {
+            for (_idx, device) in audio_devices.iter().enumerate() {
+                if idx as usize == _idx {
+                    return device.to_string();
+                }
+
+                if _idx == 0 {
+                    first_device = device.to_string()
+                }
+            }
+        }
+        Err(err) => panic!("Failed to get audio devices: {}", err),
+    };
+
+    // return first device as default, if none were matched
+    first_device
+}