preferences: revised audio tab

Cleaned up and improved the audio tab.

Fixes #1546. Kind of. There doesn't seem to be much need for a
sample rate selection for our current high-level audio system.
If we implement higher end audio backends in the future (e.g.
ASIO, JACK), those can be given a sample rate setting.

app/audio/audiomanager.cpp

@@ -28,6 +28,18 @@ namespace olive {
 
 AudioManager* AudioManager::instance_ = nullptr;
 
+QString AudioManager::GetAudioBackendName(AudioManager::Backend b)
+{
+  switch (b) {
+  case kAudioBackendQt:
+    return tr("Qt");
+  case kAudioBackendCount:
+    break;
+  }
+
+  return tr("Unknown");
+}
+
 void AudioManager::CreateInstance()
 {
   if (instance_ == nullptr) {