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

lib/filament/utils/camera_orientation.dart

@@ -0,0 +1,15 @@
+import 'package:vector_math/vector_math_64.dart' as v;
+
+class CameraOrientation {
+  v.Vector3 position = v.Vector3(0, 0, 0);
+
+  var rotationX = 0.0;
+  var rotationY = 0.0;
+  var rotationZ = 0.0;
+
+  v.Quaternion compose() {
+    return v.Quaternion.axisAngle(v.Vector3(0, 0, 1), rotationZ) *
+        v.Quaternion.axisAngle(v.Vector3(0, 1, 0), rotationY) *
+        v.Quaternion.axisAngle(v.Vector3(1, 0, 0), rotationX);
+  }
+}