Add support for Multi Window

[edgarfgp]

• Custom FabAvaloniaApp
• Make DestopApplication a CollectionBuilder
• Create a custom window with an Id so we can look up it.

Pseudocode:

new Avalonia.Window().Show() -->  Application.Windows[1]

Application(Window())

Application() {
    Window()
    Window() --> Fabulous can not add to ReadOnlyList
}

Application() {
    // This is the main window
    Window(...)

    // This is the second window
    Window(id = "secondWindow", content = ...)
}

type FabWindow() =
    inherit Window()

    member this.WindowId

type FabAvaloniaApp() =
    inherit Application()

    member this.InternalWindows = new List<Window>()    <--- this one

    override this.MainWindow = this.InternalWindows[0]

    member this.OpenWindow(id: string) =
        let window = this.InternalWindows |> List.find (fun w -> w.WindowId = id)
        window.Show()


Button("Open second window", Clicked)

match msg with
| Clicked -> Application.Current.OpenWindow("secondWindow")

[katatunix]

Yes, I need this too.