Avalonia update

[jsboige]

This is my attempt at upgrading to latest Avalonia.
There is still some work to do, but I believe I covered most breaking changes.

[BobLd]

@jsboige thanks again for working on that. unfortunately I'm getting the following error while starting this app

System.InvalidCastException
  HResult=0x80004002
  Message=Specified cast is not valid.
  Source=Panoptes
  StackTrace:
   at Panoptes.Views.Windows.SplashScreenWindow.DynamicSetters_204.DynamicSetter_2(Image , Object )
   at Panoptes.Views.Windows.SplashScreenWindow.!XamlIlPopulate(IServiceProvider , SplashScreenWindow ) in C:\Users\Bob\Source\Repos\jsboige\lean-monitor-2\Panoptes\Views/Windows/SplashScreenWindow.axaml:line 35
   at Panoptes.Views.Windows.SplashScreenWindow.!XamlIlPopulateTrampoline(SplashScreenWindow )
   at Panoptes.Views.Windows.SplashScreenWindow.InitializeComponent() in C:\Users\Bob\Source\Repos\jsboige\lean-monitor-2\Panoptes\Views\Windows\SplashScreenWindow.axaml.cs:line 36
   at Panoptes.Views.Windows.SplashScreenWindow..ctor() in C:\Users\Bob\Source\Repos\jsboige\lean-monitor-2\Panoptes\Views\Windows\SplashScreenWindow.axaml.cs:line 23
   at Panoptes.App.<LoadSplashScreen>d__3.MoveNext() in C:\Users\Bob\Source\Repos\jsboige\lean-monitor-2\Panoptes\App.axaml.cs:line 70
   at Panoptes.App.<OnFrameworkInitializationCompleted>d__2.MoveNext() in C:\Users\Bob\Source\Repos\jsboige\lean-monitor-2\Panoptes\App.axaml.cs:line 46

Also, I think the Oxyplot nugget package might need an update (not sure it was done here) oxyplot/oxyplot-avalonia#55

Happy to have a deeper look into that, I think your other PR #12 should go first though

[jsboige]

Yes, I'm sorry if that wasn't clear, but that PR is not ready to go. It took me long to get rid of compile time errors, but I suppose that is just half of the required effort... I know nothing of Avalonia, so I suppose you can help moving forward.