fspreadsheet: Read/write merged cells to biff8 (biff2 and 5 do not support this). Reader ok, but writer creates defective files... Some cosmetics and clean-up.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3541 8e941d3f-bd1b-0410-a28a-d453659cc2b4