rewrite of audio output manager for vastly improved operation

Previously the audio output manager was a "hybrid device" that acted as a pull
device for audio devices and as a push device and device proxy for the rest of
the application. This approach turned out to be flawed, particularly in the
frequent opening and closing of the audio device (extremely slow). Since we
use both pulling (for constant playback) and pushing (audio scrubbing, short
bursts of sound), it needed a similar but different approach. This approach
will switch the output device from push mode (default) to pull mode (during
playback) only when necessary resulting in far less UI lag (basically
unnoticeable now) than the previous approach.

app/audio/audiomanager.cpp

@@ -76,25 +76,17 @@ bool AudioManager::IsRefreshing()
 
 void AudioManager::PushToOutput(const QByteArray &samples)
 {
-  QMetaObject::invokeMethod(&output_manager_,
-                            "Push",
-                            Qt::QueuedConnection,
-                            Q_ARG(QByteArray, samples));
+  output_manager_.Push(samples);
 }
 
 void AudioManager::StartOutput(QIODevice *device)
 {
-  QMetaObject::invokeMethod(&output_manager_,
-                            "ConnectDevice",
-                            Qt::QueuedConnection,
-                            Q_ARG(QIODevice*, device));
+  output_manager_.PullFromDevice(device);
 }
 
 void AudioManager::StopOutput()
 {
-  QMetaObject::invokeMethod(&output_manager_,
-                            "Stop",
-                            Qt::QueuedConnection);
+  output_manager_.ResetToPushMode();
 }
 
 void AudioManager::SetOutputDevice(const QAudioDeviceInfo &info)
@@ -142,11 +134,7 @@ void AudioManager::SetOutputDevice(const QAudioDeviceInfo &info)
       abort();
     }
 
-    QMetaObject::invokeMethod(&output_manager_,
-                              "SetOutputDevice",
-                              Qt::QueuedConnection,
-                              Q_ARG(QAudioDeviceInfo, info),
-                              Q_ARG(QAudioFormat, format));
+    output_manager_.SetOutputDevice(info, format);
   }
 
   // Un-comment this to get debug information about what the audio output is doing
@@ -189,12 +177,6 @@ AudioManager::AudioManager() :
   connect(&output_manager_, &AudioHybridDevice::SentSamples, this, &AudioManager::SentSamples);
 
   output_manager_.SetEnableSendingSamples(true);
-  output_manager_.open(AudioHybridDevice::ReadOnly);
-
-  QThread* output_thread = new QThread();
-  output_thread->start();
-
-  output_manager_.moveToThread(output_thread);
 }
 
 void AudioManager::RefreshThreadDone()