use user-extended-away status icon in favour of user-idle
The freedesktop.org icon spec defines that the icon for the extended away
status is called user-idle. However, some icon theme authors interpret this
name as meaning mere idleness rather than the intended extended away semantic
of away *and* idle, creating visual confusion (extended-away contacts appearing
less away than contacts who are merely away). Some themes contain a
non-standard user-extended-away icon with the correct semantic, so this patch
makes Empathy use it in preference to user-idle if available. (Incidentally, at
the moment Telepathy itself never actually shows remote contacts as idle.)