Don't generate externalReferences tags if non externalLinks exists
API refactoring: write*() return boolean instead of integer
Fix potential crash when current sheet is not Worksheet
Eliminate some compile warnings
Refactoring: Rename OOXmlFile to AbstractOOXmlFile
Class Refactoring
Introduce new class Chartsheet, first step
Introduce CreateFlag for all the ooxmlfile
Chartsheet works now
Don't create the default "Sheet1" worksheet

Unless the user want to access it, or the user want to save an empty
workbook.