general: use IdlePriority rather than LowPriority

Fixes a number of playback stuttering and general UI lag issues by setting all
background tasks to IdlePriority rather than LowPriority. While it was assumed
LowPriority tasks would always get scheduled below NormalPriority (e.g. main
thread) tasks, it turns out this is not always the case. If the background tasks
start consuming a lot of CPU cycles, the scheduler may use "dynamic scheduling"
to schedule them above the main thread regardless leading to UI lag. This is
apparently the case for all thread priorities apart from IdlePriority, which
is allegedly a special case where threads are *only* scheduled when other
threads aren't busy ensuring the main thread stays responsive.

app/dialog/task/task.cpp

@@ -34,7 +34,7 @@ void TaskDialog::open()
   }
 
   // Create a separate thread to run this task in
-  thread_->start(QThread::LowPriority);
+  thread_->start();
 
   // Move the task to this thread
   task_->moveToThread(thread_);