Remove Webkit usage in potd data engine.

Summary:
For natgeoprovider, since the page it can be accepted by QXmlStreamReader
to get required content, we simply parse it as XML though HTML is NOT
necessarily valid XML.

For noaaprovider, QXmlStreamReader will hit an error in the middle of parsing
so we use regular expression to extract the url.

Test Plan: Manually test with plasmashell

Reviewers: mart, hein, broulik

Reviewed By: broulik

Subscribers: #plasma, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3372