Make AppBar transparent and show background image which is set to whole screen

This is supported by Scaffold now (in stable – v1.12.13+hotfix.5).

• Set Scaffold extendBodyBehindAppBar to true,
• Set AppBar elevation to 0 to get rid of shadow,
• Set AppBar backgroundColor transparency as needed.

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      extendBodyBehindAppBar: true,
      backgroundColor: Colors.red,
      appBar: AppBar(
//        backgroundColor: Colors.transparent,
        backgroundColor: Color(0x44000000),
        elevation: 0,
        title: Text("Title"),
      ),
      body: Center(child: Text("Content")),
    );
  }