Merged /branches/groupstorage

This adds a new "flat storage" type for storing single-valued properties of multi-valued groups in a more compact form than FX_CONTENT_DATA, which is now only required for "true" multi-valued properties (i.e. properties with a maximum multiplicity greater than 1).

git-svn-id: http://svn.flexive.org/repository/flexive/trunk@2895 d35d4baa-7912-0410-b0ae-edacad209e15