Same behavior for scrollbar as the desktop style

Summary:
bigger scrollbar and add a small line separator
to look consistent with D26655 and D26654

same behavior for mobile scrollbar

Test Plan:

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26664