fpspreadsheet: Clean-up of demos (sometimes the default built mode was lost). Add some more  overloaded methods to TsWorksheet having the cell as an argument instead of row/col index.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3411 8e941d3f-bd1b-0410-a28a-d453659cc2b4