Add IKeyboardInputSite.TranslateAccelerator (or like) for proper WebView2 intergration

The existing `IKeyboardInputSink.TranslateAccelerator` serves the purpose for when a container offers its child control a chance to handle a keystroke.

Sometimes, we need to do this in the opposite direction. I.e., a control asking its container if it's interested to handle a specific keystroke. To give it some practical context, please check this `WebView2` issue: https://github.com/MicrosoftEdge/WebView2Feedback/issues/468.