[Feature] Live document preview plugin

Summary:
KTextEditor plugin (to support at least both Kate & KDevelop)
which (pre)views the current state of the editing copy of a text file
in the target format in a toolview, using KParts.

For detailed introduction see
https://marc.info/?l=kwrite-devel&m=150333078206625

Reviewers: Kate, KDevelop, cullmann

Reviewed By: Kate, cullmann

Subscribers: dhaumann, progwolff, sars, cullmann, kwrite-devel

Differential Revision: https://phabricator.kde.org/D7455