fpspreadsheet: Simplify fpsgrid demo (only new/load/save functionality). More complex version of the demo is available now as demo program "spready". Add "WriteCellAvalueAsString" to TsWorksheet which guesses from the passed string whether it is a number, percentage, date, or time.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3213 8e941d3f-bd1b-0410-a28a-d453659cc2b4