LogWidget: Use FixedSizeQueue for a log messages buffer

Messages buffer is intended to be of a fixed capacity (MAX_LOG_LINES),
which cannot be achieved by std::queue unless we manually pop() extra elements.
std::queue uses std::deque internally which most likely results in allocations performed continuously.
FixedSizeQueue keeps a single buffer during its entire lifetime, avoiding any allocations except the ones
performed by stored objects.

Source/Core/DolphinQt/Config/LogWidget.h

@@ -7,9 +7,9 @@
 #include <QDockWidget>
 
 #include <mutex>
-#include <queue>
 #include <string>
 
+#include "Common/FixedSizeQueue.h"
 #include "Common/Logging/LogManager.h"
 
 class QCheckBox;
@@ -49,6 +49,9 @@ private:
 
   using LogEntry = std::pair<std::string, LogTypes::LOG_LEVELS>;
 
+  // Maximum number of lines to show in log viewer
+  static constexpr int MAX_LOG_LINES = 5000;
+
   std::mutex m_log_mutex;
-  std::queue<LogEntry> m_log_queue;
+  FixedSizeQueue<LogEntry, MAX_LOG_LINES> m_log_ring_buffer;
 };