[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: {F6287038}

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