Add Unsplash POTD Provider

Summary:
Add Unsplash Wallpaper as picture of the day provider. Unsplash provides high quality photos under public domain license.

{F7013045}

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22426