properties that are currently empty in the database instead of automatically
replacing.
feature 1312: as a matter of consistency, I have changed the replace_mode
(that was only for multiple values properties) into multiple_value_mode +
single_value_mode.
git-svn-id: http://piwigo.org/svn/branches/2.0@4459 68402e56-0260-453c-a942-63ccdbb3a9ee