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: {F5780078}

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