flutter: Rename Platform.dart to Platform_flutter.dart

Makes it consistent with the naming of all other frontends

src/flutter/dart/lib/Platform_flutter.dart

@@ -0,0 +1,8 @@
+import 'package:KDDockWidgetsBindings/src/Platform.dart'
+    as KDDockWidgetBindings;
+
+class Platform_flutter extends KDDockWidgetBindings.Platform {
+  String name() {
+    return "flutter";
+  }
+}