[RFC] Insert new tabs to the right with reduced jumpiness

As requested in https://invent.kde.org/kde/kate/merge_requests/21#note_15692, this merge request is a combination of https://invent.kde.org/kde/kate/merge_requests/21 and reverting https://invent.kde.org/kde/kate/commit/d3c3008e18342e96193d6d0dba7fcea62c2d570c.

I've found that it works well for me--better than the current default behavior. However this patch is mostly intended to facilitate discussion and is not merge-ready. I have put no effort whatsoever into polishing it and I have no doubt that it contains many bugs. :)