Add internal support for dxf format

test needed.
Documentation: Add a note for macro XLSX_NO_LIB
Don't depend on the "count" attribute

"count" is an optional attribute
Remove the private class XmlStreamWriter and XmlStreamReader
Add a warning message in all the private headers
Format: Make some private memeber public
Use namespaceUri instead of qualifiedName
Add basic conditional formatting writer support.
Add dataBar and colorScale write support
Add new private class XlsxColor
Make use of XlsxColor for ConditionalFormatting
Don't lost conditionalformatting data when edit an existing file
Remove qDebug statement
Format refactor: Don't store default properties

So that fontKey/fillKey/borderKey/formatKey will be unique for the same
formats
XlsxColor refactor