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

lib/filament/widgets/entity_controller_mouse_widget.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+
+import 'dart:async';
+
+import 'package:flutter_filament/filament/entities/entity_transform_controller.dart';
+
+///
+/// A widget that translates mouse gestures to zoom/pan/rotate actions.
+///
+class EntityTransformMouseControllerWidget extends StatelessWidget {
+  final EntityTransformController? transformController;
+  final Widget? child;
+
+  EntityTransformMouseControllerWidget(
+      {Key? key, required this.transformController, this.child})
+      : super(key: key);
+
+  Timer? _timer;
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(
+        builder: (BuildContext context, BoxConstraints constraints) {
+      return Listener(
+          onPointerDown: (event) {
+            if (kPrimaryMouseButton & event.buttons != 0) {
+              transformController?.mouse1Down();
+            }
+          },
+          onPointerUp: (event) {
+            if (kPrimaryMouseButton & event.buttons != 0) {
+              transformController?.mouse1Up();
+            }
+          },
+          onPointerHover: (event) {
+            _timer?.cancel();
+            if (event.position.dx < 10) {
+              _timer = Timer.periodic(const Duration(milliseconds: 17), (_) {
+                transformController?.look(-30);
+              });
+            } else if (event.position.dx > constraints.maxWidth - 10) {
+              _timer = Timer.periodic(const Duration(milliseconds: 17), (_) {
+                transformController?.look(30);
+              });
+            } else {
+              transformController?.look(event.delta.dx);
+            }
+          },
+          child: child);
+    });
+  }
+}