Update KOrganizer POTD plugin for current Wikipedia API
The plugin is currently defunc, as its code seems written against a Wikipedia API of ancient times.
This patch makes things working again, as the old code based on some (updated it seems) sample code from Wikipedia itself at https://www.mediawiki.org/wiki/API:Picture_of_the_day_viewer. See for current week:
Additionally fancies some improvements like a small cache to save some repeated requests when switching days /weeks within a month,
Beyond being perfect, but better than being defunc as of now. And having something working again might bring users to shape the plugin into usage optimized behaviour.
Ideally would be done against current release branch, but no local development setup to backport. Unless there is big demand, I would keep targetting only 21.12 as first release.