Change default save path to ~/Videos

Summary:
BUG: 385367

Changed default save path to ~/Videos, which seems like a more humane starting location than /var/tmp.

Test Plan: Can't test; don't have any machines with optical drives. Would appreciate some rudimentary testing from anyone with appropriate hardware.

Reviewers: #kde_applications, lesliezhai, ltoscano

Reviewed By: lesliezhai

Subscribers: ltoscano

Differential Revision: https://phabricator.kde.org/D8231