Code refactoring: add saveToXmlData() to Theme
Relationships code refactoring: Don't use static member
Coding refactoring: Make worksheet relations part of the worksheet

sheet1.xml
_rels/sheet1.xml.rels
Fix compile error under maxos
Fix variable uninitialized warning under macos/clang

warning: variable 't' is used uninitialized whenever 'if' condition is false
[-Wsometimes-uninitialized]
Don't generate .rel file for worksheet if its RelationShips is empty
Add const to the saveAs* members, step 1
Relations: Fix unit test

Introduced by SHA: 45a86acceea9a1bfb4499f970e698988e7d9d87c
Code refactoring: Clean up code of QXlsx::Package

We should merge the class Package with the class Document
Code refactoring: Remove private class QXlsx::Package
Add const to the saveAs* members, step 2
Code refactoring: rename private members of Worksheet
Fix a silly bug
Don't lost hyperlinks when edit an exist xlsx file
Code refactoring: relationships of workbook