Extend KTextEditor-plugin integration to activate VI-Mode commands

Summary:

• currently opening of documents is not connected to KTextEditor
• enable switching of tabs by :bn, :bp, :tabn, ...
• fix closing of unsaved documents via :tabc

FIX: 371235

Test Plan:

• open a document with :e filename
• tab switching requires D8710

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8712