IRC accounts: make labels the same width.
IRC accounts: make labels the same width.

This addresses Nick's concern on
https://bugzilla.gnome.org/show_bug.cgi?id=654552

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