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

lib/filament/entities/gizmo.dart

@@ -0,0 +1,72 @@
+import 'dart:ui';
+
+import 'package:vector_math/vector_math_64.dart';
+
+import '../filament_controller.dart';
+
+class Gizmo {
+  final FilamentEntity x;
+  Vector3 _x = Vector3(0.1, 0, 0);
+  final FilamentEntity y;
+  Vector3 _y = Vector3(0.0, 0.1, 0);
+  final FilamentEntity z;
+  Vector3 _z = Vector3(0.0, 0.0, 0.1);
+
+  final FilamentController controller;
+
+  FilamentEntity? _activeAxis;
+  FilamentEntity? _activeEntity;
+  bool get isActive => _activeAxis != null;
+
+  Gizmo(this.x, this.y, this.z, this.controller) {
+    controller.pickResult.listen(_onPickResult);
+  }
+
+  Future _reveal() async {
+    await controller.reveal(x, null);
+    await controller.reveal(y, null);
+    await controller.reveal(z, null);
+  }
+
+  void translate(Offset offset) async {
+    late Vector3 vec;
+    if (_activeAxis == x) {
+      vec = _x;
+    } else if (_activeAxis == y) {
+      vec = _y;
+    } else if (_activeAxis == z) {
+      vec = _z;
+    }
+    await controller.queuePositionUpdate(_activeEntity!, offset.dx * vec.x,
+        -offset.dy * vec.y, -offset.dx * vec.z,
+        relative: true);
+  }
+
+  void reset() {
+    _activeAxis = null;
+  }
+
+  void _onPickResult(FilamentPickResult result) async {
+    if (result.entity == x || result.entity == y || result.entity == z) {
+      _activeAxis = result.entity;
+    } else {
+      attach(result.entity);
+    }
+  }
+
+  void attach(FilamentEntity entity) async {
+    print("Attaching to $entity");
+    _activeAxis = null;
+    _activeEntity = entity;
+    await _reveal();
+    await controller.setParent(x, entity);
+    await controller.setParent(y, entity);
+    await controller.setParent(z, entity);
+  }
+
+  void detach() async {
+    await controller.hide(x, null);
+    await controller.hide(y, null);
+    await controller.hide(z, null);
+  }
+}