[Feature]Add ability to persist GridSplitter position and restore it in the next session

### Describe the problem

There is no easy way to query the current splitter position and restore it alter. There is no single Position property and no event firing when the splitter is moved. One can hook up resize event of one of the controls and then query row/column definitions for width/height, but GridSplitter does it anyway; need a way to expose it for external consumption.

When restoring the previously saved position, the logic in HorizontalMove / VerticalMove can be replicated externally, but this really needs to be exposed by the control itself.

### Describe the solution

Expose Position property and an event firing when it changes.

### Alternatives

_No response_

### Additional info

_No response_

### Help us help you

None