Add template for a ktexteditor plugin

Summary:
Inspired by ktexteeditor template from kapptemplate.

Keeping the template with the sources they are based on
helps both discovery and also maintenance.

Reviewers: Frameworks, KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: cullmann, kwrite-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6803