[Feature] WCT GridSplitter element

[jamesmcroft]

Description

Add support for the Windows Community Toolkit GridSplitter element.

The implementation should support the ability to select a GridSplitter, hold it, and change the width/height of the elements.

[jamesmcroft]

Think this one might be a bit ambitious for Hacktoberfest. The GridSplitter control is exposed correctly as a control in the UI Automation tree but doesn't provide much in the way of meaningful attributes.

It may be that a custom AutomationPeer needs to be added to that specific control in the Windows Community Toolkit in order to be accommodated here.