Add scrollbuttons for tabs when tabbar is full

When there are too many tabs to be shown in the window, a scrollbar
appears in the tabbar (this is not great usability-wise, but still
beat window auto-resizing, particularly when the window is already
maximized)

This is part of this merge request
https://invent.kde.org/kde/konsole/merge_requests/25
I'll leave changing TextElideMode for later review.

CCBUG: 403059
FIXED-IN: 19.08.1
(cherry picked from commit f79ee976)