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/audiooutputdeviceproxy.cpp

@@ -0,0 +1,39 @@
+#include "audiooutputdeviceproxy.h"
+
+#include "audiobufferaverage.h"
+
+AudioOutputDeviceProxy::AudioOutputDeviceProxy() :
+  device_(nullptr),
+  send_averages_(false)
+{
+}
+
+void AudioOutputDeviceProxy::SetDevice(QIODevice *device)
+{
+  device_ = device;
+}
+
+void AudioOutputDeviceProxy::SetSendAverages(bool send)
+{
+  send_averages_ = send;
+}
+
+qint64 AudioOutputDeviceProxy::readData(char *data, qint64 maxlen)
+{
+  if (device_) {
+    qint64 read_count = device_->read(data, maxlen);
+
+    if (send_averages_ && read_count > 0) {
+      emit ProcessedAverages(AudioBufferAverage::ProcessAverages(data, static_cast<int>(read_count)));
+    }
+
+    return read_count;
+  }
+
+  return 0;
+}
+
+qint64 AudioOutputDeviceProxy::writeData(const char *data, qint64 maxSize)
+{
+  return -1;
+}