Does not honour PREF parameters

[da4089]

Import from: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=695941

vobject.adr seems to be the first entry of vobject.adr_list,
unconditionally, even if the second entry has a higher-ranked (=
lower) preference parameter, e.g.

ADR;PREF=2:foo
ADR;PREF=1:bar

In this case, vobject.adr should really correspond to the "bar"
entry.

Along similar lines, it would be good if there were a filter
function for adr_list and other type entries that can be duplicated
and have PREF parameters.