fpspreadsheet: Fix absolute/relative cell reference issues when reading shared formulas. Update BIFFExplorer for shared formulas. Implement reading of shared formulas for biff5 (nothing to be done for biff2 which does not support shared formulas).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3490 8e941d3f-bd1b-0410-a28a-d453659cc2b4