refactor Windows classes to separate EGL/WGL/Backing Window

windows/utils.h

@@ -0,0 +1,20 @@
+// This file is a part of flutter_native_view
+// (https://github.com/alexmercerind/flutter_native_view).
+//
+// Copyright (c) 2022, Hitesh Kumar Saini <saini123hitesh@gmail.com>.
+// All rights reserved.
+// Use of this source code is governed by MIT license that can be found in the
+// LICENSE file.
+
+#include <dwmapi.h>
+
+#include <cstdint>
+
+namespace flutternativeview {
+
+RTL_OSVERSIONINFOW GetWindowsVersion();
+
+void SetWindowComposition(HWND window, int32_t accent_state,
+                          int32_t gradient_color);
+
+}  // namespace flutternativeview