BindableCollection races against any call which adds/removes elements before it

^{Originally posted by **Yoooi0** November 21, 2021}
There seems to be an issue with thread safety in BindableCollection.

When two threads modify the collection at the same time, one calling [ClearItems](https://github.com/canton7/Stylet/blob/5e97f9e1ea8c45e9d7033e6e7c7ea8ce41c3420a/Stylet/BindableCollection.cs#L216), the other calling [RemoveItem](https://github.com/canton7/Stylet/blob/5e97f9e1ea8c45e9d7033e6e7c7ea8ce41c3420a/Stylet/BindableCollection.cs#L203), it can cause `OnCollectionChanging` to throw an exception.

```
System.Reflection.TargetInvocationException: "An error occurred while dispatching a call to the UI Thread"

Inner Exception
ArgumentOutOfRangeException: "Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')"

at Stylet.Execute.OnUIThreadSync(Action action)
at Stylet.BindableCollection`1.RemoveItem(Int32 index)
at System.Collections.ObjectModel.Collection`1.Remove(T item)
```

Example timeline:
* Create BindableCollection with 1 item
* Thread 1 calls `collection.ClearItems()`
* Thread 1 enters `Execute.OnUIThreadSync`
* Thread 2 calls `collection.Remove(firstItem)`, this calls `RemoveItem` with `index` argument set to `0`
* Thread 2 waits for thread 1
* Thread 1 clears the collection by calling `base.ClearItems()`
* Thread 2 enters `Execute.OnUIThreadSync`
* Thread 2 throws exception when calling `OnCollectionChanging` because the item at index 0 does not exist anymore