Preset file name of archive in CreateDialog

Summary:
The file name can be set explicitly by --add-to option or
detected automatically with --add option. Set it in the
dialog for the convenience of users.

Reviewers: #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: elvisangelaccio, kde-utils-devel, #ark

Tags: #ark

Differential Revision: https://phabricator.kde.org/D18152