[Slideshow] increase default interval from 10 seconds to 15 minutes

Summary:
10 seconds isn't the best default value for slideshows. 15 minutes is more reasonable, and easier on battery life for laptops, too.

For comparison: macOS and Windows both default to 30 minutes.

Test Plan:

Reviewers: Plasma, broulik, mart, VDG, abetts, davidedmundson

Reviewed By: Plasma, VDG, abetts, davidedmundson

Subscribers: abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15785