Run moodbar pipeline threads at idle CPU and IO priority. Fixes issue 3644.

src/moodbar/moodbarloader.cpp

@@ -111,7 +111,7 @@ MoodbarLoader::Result MoodbarLoader::Load(
   }
 
   if (!thread_->isRunning())
-    thread_->start();
+    thread_->start(QThread::IdlePriority);
 
   // There was no existing file, analyze the audio file and create one.
   MoodbarPipeline* pipeline = new MoodbarPipeline(url);

src/moodbar/moodbarpipeline.cpp

@@ -23,6 +23,7 @@
 
 #include "core/logging.h"
 #include "core/signalchecker.h"
+#include "core/utilities.h"
 
 bool MoodbarPipeline::sIsAvailable = false;
 
@@ -74,6 +75,8 @@ GstElement* MoodbarPipeline::CreateElement(const QString& factory_name) {
 void MoodbarPipeline::Start() {
   Q_ASSERT(QThread::currentThread() != qApp->thread());
 
+  Utilities::SetThreadIOPriority(Utilities::IOPRIO_CLASS_IDLE);
+
   if (pipeline_) {
     return;
   }