Show the scrollbar only when needed

Summary:
It hides the scrollbar when there isn't enough lines to scroll.
This also affects KParts like Yakuake or Dolphin and others.

Test Plan:

Reviewers: hindenburg, Konsole, VDG

Reviewed By: hindenburg, Konsole, VDG

Subscribers: konsole-devel, mglb, zzag, Pitel, ngraham, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D11843