SButton does not release mouse capture if it becomes disabled before MouseUp event occurs

It appears that if a slate widget becomes disabled, the MouseUp events won't fire. As a side effect, a mouse capture will not be released if something causes the button to become disabled before the mouse is released.

1. Create a new widget with a button
2. Set the button to disable itself on press
3. Press the button and observe that it does not release mouse capture

Editor freezes when starting sequence recording from SButton