fpspreadsheet: Fix remaining utf8 issues when reading/writing rpn string formulas. Fix  BIFFExplorer issues when displaying biff2 formulas. Excel complains about data loss when reading a biff2 file containing a calculated formula having a string result - not fixed yet.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3266 8e941d3f-bd1b-0410-a28a-d453659cc2b4