fpspreadsheet: Improved reading of shared strings in xlsx files (see http://forum.lazarus.freepascal.org/index.php/topic,25624.msg155857.html#msg155857)
Add unit xlsconst.pas (will be needed for string formula support)

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3503 8e941d3f-bd1b-0410-a28a-d453659cc2b4