[potd dataengine] De-pimplify potd providers for more simple code
Summary:
There is no need to use pimpl for non-public-API classes.
While touching the code, also change to use consistently
modern signal-slot connections.
Test Plan: All plugins still build, load & work as before.
Reviewers: xuetianweng
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11957