Add svn_string_t property values to svn_wc_conflict_description2_t.

For historical reasons these values are available in temporary files.
However, svn_string_t is the canonical representations for property values
and can be easier to work with when implementing conflict resolvers.

* subversion/include/svn_wc.h
  (svn_wc_conflict_description2_t): Add prop_value_base, prop_value_working,
   prop_value_incoming_old, prop_value_incoming_new.

* subversion/libsvn_wc/conflicts.c
  (generate_propconflict, read_prop_conflict_descs): Set property values.

* subversion/libsvn_wc/util.c
  (svn_wc_conflict_description2_dup): Copy property values if set.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619122 13f79535-47bb-0310-9956-ffa450edef68