fpspreadsheet: Move log list from reader/writer to workbook for easier access. Add unit test for error logging (in internaltests). Improve OOXML writer to begin writing rows only from the first existing row, not from 0. (The ODS writer still does this, therefore, it creates a huge file for the error logging test - this test is currently deactivated for ods).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3446 8e941d3f-bd1b-0410-a28a-d453659cc2b4