fpspreadsheet: Fix circular unit reference bug (thanks, Michael). Add demo_sorting.pas unit missing from last commit. Some improvements with multi-key sorting, but still in error sometimes (--> sorting code is still experimental).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3678 8e941d3f-bd1b-0410-a28a-d453659cc2b4