fpspreadsheet: Fix crash of BIFF Explorer when viewing RK records with negative numbers. Add info page for IXFE record (BIFF2 only). Fix info page for COLWIDTH records (BIFF2).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3081 8e941d3f-bd1b-0410-a28a-d453659cc2b4