version: derive from git tag/commit hash; drop audio playback debug logs

Version is no longer hardcoded: at configure time CMake uses the tag
name when HEAD is exactly on a tag (leading "v" stripped), otherwise
the first 8 hex digits of the commit hash. When git is unavailable
(e.g. source tarball) it falls back to the contents of version.txt,
which is now the single place to bump the release version.

Also remove the temporary qDebug() flood in the audio playback path
(ViewerWidget::QueueNextAudioBuffer / ReceivedAudioBufferForPlayback,
AudioManager::PushToOutput).

app/audio/audiomanager.cpp

@@ -93,11 +93,6 @@ int InputCallback(const void *input, void *output, unsigned long frameCount,
 bool AudioManager::PushToOutput(const AudioParams &params,
 								const QByteArray &samples, QString *error)
 {
-	qDebug() << "AudioManager::PushToOutput: device=" << output_device_
-			 << "sample_rate=" << params.sample_rate()
-			 << "channels=" << params.channel_count()
-			 << "bytes=" << samples.size();
-
 	if (output_device_ == paNoDevice) {
 		if (error)
 			*error = tr("No output device is set");