ValEdit: Fix FItemProps getting out of sync when Inserting, Deleting or Assigning strings,
which caused a crash on Delete(), and memory leaks.
Fixes issue #0023851


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40209 4005530d-fff6-0310-9dd1-cebe43e6787f