Fixing gestures conflicts and adding a gestures example

doc/examples/gestures/lib/main.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+import 'package:flame/game.dart';
+
+void main() {
+  final game = MyGame();
+  runApp(game.widget);
+}
+
+class MyGame extends Game {
+  final _whitePaint = Paint()..color = const Color(0xFFFFFFFF);
+  final _bluePaint = Paint()..color = const Color(0xFF0000FF);
+  final _greenPaint = Paint()..color = const Color(0xFF00FF00);
+
+  Paint _paint;
+
+  Rect _rect = const Rect.fromLTWH(50, 50, 50, 50);
+
+  MyGame() {
+    _paint = _whitePaint;
+  }
+
+  @override
+  void onTap() {
+    _paint = _paint == _whitePaint ? _bluePaint : _whitePaint;
+  }
+
+  @override
+  bool useDoubleTapDetectors() => true;
+
+  @override
+  void onDoubleTap() {
+    _paint = _greenPaint;
+  }
+
+  @override
+  bool usePanDetectors() => true;
+
+  @override
+  void onPanUpdate(DragUpdateDetails details) {
+    _rect = _rect.translate(details.delta.dx, details.delta.dy);
+  }
+
+  @override
+  void update(double dt) {}
+
+  @override
+  void render(Canvas canvas) {
+    canvas.drawRect(_rect, _paint);
+  }
+}