fpspreadsheet: Remove string-based writing code of ods files --> significant speed improvement particularly for large grids (20000x100 cells 120 sec (old) --> 7 sec (new)).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3311 8e941d3f-bd1b-0410-a28a-d453659cc2b4