Port internal render plugins to HtmlWriter

Summary:
The next steps are using streaming API also for the nested Grantlee
invocations and replacing DefaultRendererPrivate* in the render plugin
API.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8102