Fix Unsplash POTD not updating

Summary:
The bug only affect Unsplash POTD provider. If you choose Unsplash POTD, the wallpaper won't update daily.

Previous patch for Unsplash Wallpapers changed the identifier format, for example, `unsplash:123456` is not a fixed date identifier. So we can not simply detect whether an identifier contains date by looking for `:`. This patch detect date string with regular expression.

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22925