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
wip