fpsreadsheet: Introduce "virtual writing mode" where the writer does not get its data from the spreadsheet, but from an event ("OnNeedCellData"). Introduce stream switching for xlsxooxml. Both feature allow to write HUGE spreadsheet files. Add example "test_virtualmode".

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3306 8e941d3f-bd1b-0410-a28a-d453659cc2b4