Reproducer:
1) Connect to freenode
2) Wait for nickserv to spam with "this nickname is registered" /
   "you're now identified for ..."
3) While polari is focused, click on the notification from the shell,
   opening a private conversation with nickserv
(Don't scroll or switch to another app)
4) Observe that even though polari is focused, the dot in the shell
   is still present, because the messages were not acked
5) Switch to another channel
6) Switch to another app and back to polari
   (This is a good moment to check that indeed messages were not
   acked in d-feet)
7) Switch to nickserv
8) Observe that still messages where not acked - nickserv's ChatView
   still thinks polari is not focused

Alternatives to this patch:
1) Stop caching _toplevelFocus and replace with an accessor that
   queries the widget flags
2) Maybe initialize _toplevelFocus to true instead of false?
   I don't know if the opposite case works, maybe it's harder to
   reproduce.