make quotedHTML ObjectTreeSource independend

Summary:
All usage of ObjectTreeSource is renderer specific.
Move it up the stack to the RendererContext.

Test Plan: run tests inside messagelib/kdepim-addonos

Reviewers: vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8889