Windows: Use common code paths for default font families.

Factor out function to determine the font family for a style
hint and use that for the FreeType font database as well.

Task-number: QTBUG-39961
Change-Id: Ic5ec0928c7994b61e4d2b10d829826658b1aa1d5
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>