Add convenient function saveToXmlData() for worksheet
Refactor QXlsx::Styles and QXlsx::Format
Make that QXlsx::Document is the only subclass of QObject
wip: Add read function for workbook
Fix issue 4: Fix background color of solid pattern

For a solid fill, Excel reverses the role of foreground and
background colours
Fix issue 3: QXlsx::Worksheet::setColumn should use 0-based index