Fix: refactoring appButton

src/game/lib/fullscreen_button.js

@@ -28,7 +28,10 @@ class FullscreenButton {
 			"#333"
 		);
 
-		let button = new RoundRectButton(setting, "", () => this.clickEvent());
+		let button = new RoundRectButton(
+			setting, RoundRectButton.NONE_ICON, RoundRectButton.NONE_BUTTON_TEXT,
+			() => this.clickEvent()
+		);
 		button.move(this.game.world.width - 30, 30);
 
 		let icon_fullscreen = game.add.sprite(0, 0, 'icon_fullscreen');