Additional style name/type properties

Currently when writing QTextDocument to ODT all styles are saved as "autostyles" (in content.xml file). Therefore these styles are not visible when ODT is opened in any office application. Therefore it is difficult to reformat resulting file by applying some style changes.

It is possible and not too difficult to extend QTextOdfWriter class to write some styles to styles.xml file. But in order to do that some additional properties are required in QTextFormat class:
1. QString style name - to represent name of style as used/visible in resulting odt file (possibly but not necessarily splitting it to "style name" and "display style name" as defined in ODF format).
2. enum style type - with values say "auto" (style saved in content.xml), "style" (style saved in style.xml).