Edited Issue: Multiline TextEditor keyboard focus on dropdown [21205]

A
```
Dispatcher.BeginInvoke(new Action(()=>_textBox.Focus()), DispatcherPriority.Background);
```
needs to be done when the Popup opens in the MultuLineTextEditor controls. We need to wait for the popup action to be over.

v2.4 of the toolkit will focus on the content of the popup when the MultiLineTextEditor opens.