Previously this project was spewing loads of
```
Severity Code Description Project File Line Suppression State
Warning CS1701 Assuming assembly reference 'System.Runtime, Version=4.0.20.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'Avalonia.Controls' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy Avalonia.Input.UnitTests(net461) D:\projects\Avalonia\tests\Avalonia.Input.UnitTests\CSC 1 Active`
```
Warnings. Not sure why this was, but updating the `csproj` to be in the same format as the other unit tests stops these warnings and reduces the warnings for a solution build from 5755(!) to 238.
Added an `IDataTemplateHost` interface with a `IsDataTemplatesInitialized` property to prevent the need for allocating empty `DataTemplates` collections for many controls.
To do this needed to change behavior a little in that now binding errors update the target. Previously in the case of a binding error at the first node in the binding chain, we were converting the `BindingNotification` to `UnsetValue` which had the effect of updating the target value. Now we're passing the `BindingNotification` back, we need to make sure this happens. I believe this is the right thing to do as the behavior should be the same no matter where in the binding chain the error occurs. Data validation errors continue to not update the target.
1. Fixed some tests to expect `BindingNotification`s to be returned on a broken binding chain.
2. Changed logging of `BindingNotification`s - log at `Warning` level (instead of `Error`) except when the binding chain is broken at the root, in which case log at `Information` level. Do this to prevent flooding the output window when initializing.
Nikita Tsukanov
Steven Kirk
Jeremy Koritzinsky
Dan Walmsley
Eli Arbel