Add splash screen

[jpnurmi]

It's an empty MaterialApp+Scaffold+WindowTitleBar with Yaru theme to bring something minimal with a titlebar visible as soon as possible on app startup.

[d-loose]

Very nice, but it's completely unnoticeable in the release version:

https://user-images.githubusercontent.com/113362648/207390342-6e6e8e5f-3e26-414e-be1c-534fe09b486f.mp4

and the title bar throws an exception

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
#0      State.setState (package:flutter/src/widgets/framework.dart:1134)
#1      _TitleBarLayoutState._syncWindowState.<anonymous closure> (package:title_bar/src/window_title_bar.dart:82)
<asynchronous suspension>
#2      Future.wait.<anonymous closure> (dart:async/future.dart:522)
<asynchronous suspension>

[jpnurmi]

Very nice, but utterly useless! :rofl:

The exception is interesting. It's not clear to me when the result of MethodChannel.invokeMethod() can be null regardless that the platform plugin never returns null for this method.

[jpnurmi]

I'll get back to this after:

• 291

• ubuntu/yaru_widgets.dart#455