Added a required to GameWidget and updated the docs (#638)

* Added a required to GameWidget and updated the docs

* Removed entry from CHANGELOG

CHANGELOG.md

@@ -23,6 +23,7 @@
  - Added `Composition`, allows for composing multiple images into a single image.
  - Move files to comply with the dart package layout convention
  - Fix gesture detection bug of children of `PositionComponent`
+ - The `game` argument on `GameWidget` is now required
 
 ## 1.0.0-rc5
  - Option for overlays to be already visible on the GameWidget

doc/game.md

@@ -27,14 +27,17 @@ class MyGameSubClass extends Game {
     
   
 main() {
+  final myGame = MyGameSubClass();
   runApp(
     GameWidget(
-      game: MyGameSubClass(),
+      game: myGame,
     )
   );
 }
 ```
 
+**Note:** Do not instantiate your game in a build method. Instead create an instance of your game and reference it within your widget structure. Otherwise your game will be rebuild every time the Flutter tree gets rebuild.
+
 It is important to notice that `Game` is an abstract class with just the very basic implementations of the gameloop.
 
 As an option and more suitable for most cases, there is the full-featured `BaseGame` class. For example, Forge2D games uses `Forge2DGame` class;

lib/src/game/game_widget.dart

@@ -99,7 +99,7 @@ class GameWidget<T extends Game> extends StatefulWidget {
   /// ```
   const GameWidget({
     Key key,
-    this.game,
+    @required this.game,
     this.textDirection,
     this.loadingBuilder,
     this.errorBuilder,