Summary:
after a major cleanup of all the deprecated/unused API/code.

What's left is:
 * automatic resizing of tabs
 * double-click in empty area to request a new tab
 * context menu on tab convenience API
 * some advanced DnD code, not fully working right now though

These features were initially written for Konqueror, it makes
sense to me that they are kept and moved here.

Test Plan: Running konqueror and opening tabs

Reviewers: stefanocrocco, vkrause

Reviewed By: vkrause

Maniphest Tasks: T11559

Differential Revision: https://phabricator.kde.org/D23841