[window-list] Purge alert-list when an alerting window receives focus.

Hi again!

Sorry to be the bearer of bad news again, but it's still not working without annoying glitches. By running tests/interactive/urgentwindow.py, removing focus from the alerting window and switching back and forth between workspaces I am able to produce duplicate alert placeholders, and clicking on a placeholder leaves zombie placeholders behind.

This patch seems to fix these glitches. Hopefully for the last time.
-- 
Per