roster-window: watch for TpAccount:connection changes
roster-window: watch for TpAccount:connection changes

The account being connected isn't longer enough to ensure that its
TpConnection is defined.

https://bugzilla.gnome.org/show_bug.cgi?id=667743

Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets