Remove DefaultRendererPrivate from the render plugin interface

Summary:
Instead, add a RenderContext class for this purpose. This will likely grow
over time as we move more of the internal rendering and external plugins
to this API, but for now it gets the job done of isolating us from private
implementation details.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8104