QmlDesigner.PropertyEditor: Fixing colouring for FontComboBox

Task-number: QTCREATORBUG-12027
Change-Id: Ic5ad0e16a7d60dacc89fa4c226e30a40a45be9db
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>