PanelConfiguration: fix clicking position preview and using arrow keys
When you click the "Set Position…" button, the buttons for changing the position appear on screen edges, and you can also use the arrow keys to change the position.
However when you click on the preview image above the button, the buttons for changing position appear on screen, but the arrow key shortsuts do not work. This is because the key handlers only live on the button, which does not have focus after you click on the preview. image.
To fix this, force focus to the button when clicking on the preview. The preview doesn't accept focus anyway so this shouldn't be a problem.