QtQuick.Dialogs FontDialog: support keyboard navigation

[ChangeLog][QtQuick.Dialogs] FontDialog: support keyboard navigation

Task-number: QTBUG-39365
Change-Id: Ic4cdbe6643be2fc66dbb23873e780a1f1f88a410
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>