WinUI3 example: doesn't refresh

[adrianhall]

Reported by @trydalch

The WinUI3 (and potentially the WPF) version of the TodoApp does not refresh properly.

[trydalch]

I believe it's just the WinUI, and the issue is related to the bindings. The underlying data is correct, but the UI isn't updated.

I actually added a fix in my PR for the initial Uno Sample (I'm discovering it may need more work for other platforms like iOS and Android) because Uno uses WinUI when targeting windows, so it had the same problem as the WinUI sample app. I fixed it for both WinUI and Uno.

The fix was three-fold:

1. Create the TodoItemViewModel that is an ObservableObject and change the type of the Items list to that. This properly raises property changed events.
2. Added Mode=TwoWay,UpdateSourceTrigger=Explicit to the Checkbox bindings.
3. Sorted the TodoItems. When it was unsorted, WinUI (and Uno) would not refresh the list UI properly and checkboxes would show checked that shouldn't and vice versa.

See: #137

[adrianhall]

Got it - I'm not actually seeing that, but I believe you; especially the sorting issue. I'll check in the fixes for the WinUI3 change soon.

Please remove the WinUI3 and WPF fixes from your PR - let's keep it focused on just the Uno sample.

[trydalch]

PR updated and removed all WPF and WinUI3 changes.