fpspreadsheet: Initial version of showing merged cells in TsWorksheetGrid. Fix typo of xlsx extension in FileOpen dialog of fpsgrid demo (see http://forum.lazarus.freepascal.org/index.php/topic,25624.msg157029.html#msg157029)

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3543 8e941d3f-bd1b-0410-a28a-d453659cc2b4