fpspreadsheet: Rewrite all WriteXXXX methods of TsWorksheet depending on row and column indexes as functions to return the pointer to the cell created/modifed.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3426 8e941d3f-bd1b-0410-a28a-d453659cc2b4