DolphinQt: Add function to set a QWidget's window decorations to dark ones on Windows.

Source/Core/DolphinQt/QtUtils/SetWindowDecorations.h

@@ -0,0 +1,9 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+class QWidget;
+
+// Changes the window decorations (title bar) to dark if the user uses dark mode on Windows.
+void SetQWidgetWindowDecorations(QWidget* widget);