add (very rough) gizmo, restructure Dart package into library, add EntityListWidget

lib/filament/utils/using_pointer.dart

@@ -0,0 +1,10 @@
+import 'dart:ffi';
+
+import 'package:ffi/ffi.dart';
+
+final allocator = calloc;
+
+void using(Pointer ptr, Future Function(Pointer ptr) function) async {
+  await function.call(ptr);
+  allocator.free(ptr);
+}