closeDialog: Untrack chrome when window loses focus

On X11, reactive chrome must be added to the input region in order to work as expected. However that region works independently from any window stacking, with the result that the unresponsive-app dialog currently blocks all input in the "covered" area, even in windows stacked above the unresponsive window.

The correct fix would be to track the unobscured parts of the dialog and set the input region from that, but that's quite cumbersome. So instead, only track chrome when the corresponding window is focused (or the dialog itself of course).

Fixes #273 (closed)