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).

[1] https://phabricator.kde.org/D27735

CCBUG: 420287