Ticket #2913: Realtime background update marker no longer triggers false positives...

Ticket #2913: Realtime background update marker no longer triggers false positives when clearing the marker on switching away from the tab in Firefox 4

We were clearing the counter on the window title in the blur event, which gets fired *after* we switch away, thus triggering Firefox to mark the tab as updated again.
Clearing the counter on *focus* instead avoids this, and keeps the counter out of the way as well.