DolphinQt: Remove another usage of QFontMetrics::width

QFontMetrics::width breaks building with CMake on Windows,
due to a deprecation warning which gets promoted to an error.

Source/Core/DolphinQt/QtUtils/FontMetricsHelper.h

@@ -0,0 +1,20 @@
+// Copyright 2020 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <utility>
+
+#include <QFontMetrics>
+
+// This exists purely to get rid of deprecation warnings while still supporting older Qt versions
+template <typename... Args>
+int FontMetricsWidth(const QFontMetrics& fm, Args&&... args)
+{
+#if QT_VERSION >= QT_VERSION_CHECK(5, 11, 0)
+  return fm.horizontalAdvance(std::forward<Args>(args)...);
+#else
+  return fm.width(std::forward<Args>(args)...);
+#endif
+}