fpspreadsheet: Add support for diagonal border lines. Implemented for BIFF8 and ods (BIFF2 and 5 do not support diagonal borders, OOXML reader currently does not have border support). Test cases adapted. "spready" adapted to show diagonal borders.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3400 8e941d3f-bd1b-0410-a28a-d453659cc2b4