Replaced Application.Current.MainPage with the IoC variant.

[EGoverde]

The Application.Current.MainPage is marked for deprecation and will be removed in future MAUI releases. This PR replaces this with the introduced IWindowCreator.

In this implementation it will create a Window when requested and sets the Shell from the Nucleus initializer as the Page.

It is possible to override / subclass this service in advanced scenarios. For Windows, or other possibly configurations where multiple windows are possible, this could use further improvements.

See:

• https://github.com/dotnet/docs-maui/pull/1819
• https://learn.microsoft.com/en-us/dotnet/maui/whats-new/dotnet-9?view=net-maui-8.0#mainpage