Update for Avalonia 12

[IhateTrains]

Changes based on https://docs.avaloniaui.net/docs/avalonia12-breaking-changes.

• Replaced Avalonia.ReactiveUI with ReactiveUI.Avalonia.
• Removed Avalonia.Diagnostics.
• Fixed the following MissingMethodException by updating RelativeSource syntax from positional to named mode:

System.MissingMethodException: Method not found: 'Void Avalonia.Markup.Xaml.MarkupExtensions.RelativeSourceExtension..ctor(Avalonia.Data.RelativeSourceMode)'.
   at CompiledAvaloniaXaml.!AvaloniaResources.Populate:/Themes/Generic.xaml(IServiceProvider, Styles)
   at CompiledAvaloniaXaml.!AvaloniaResources.Populate:/Themes/Generic.xaml(IServiceProvider, Styles)
   at CompiledAvaloniaXaml.!AvaloniaResources.Build:/Themes/Generic.xaml(IServiceProvider)

The Samples project works:

[kayoub5]

@CreateLab Any chance to get those changes releases?