kcm_fonts: Make the font selection dialog select the correct "Regular"-like style
Due to KConfig dropping QFont styleName property (for "Regular"-like font styles, see [1] for more details), the font selection dialog invoked by the KCM could end up selecting the wrong style; this change sets the appropriate "Regular" style on the QFont object before invoking the font selection dialog to fix/workaround the issue.
Note that in Plasma master branch the issue is handled differently, since we switched from QFontDialog to KFontChooserDialog (the latter has that logic built-in).