Revert the QJSValue parameter type in UIDelegate

The mapping of var signal parameters to QJSValue was reverted
in qtdeclarative, commit 605777e6cf557dbaf2835c63562bf3fe1620ee45.

Change the FilePickerController::accepted() slot parameter type from
QJSValue to QVariant in UIDelegate.

Task-number: QTBUG-44307
Change-Id: I373b37b6eb4c32e91fd92701ecad685eab9cb804
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>