show a thin separator between view and scrollbar

Summary:
This look makes listviews look way better, not having the selected
items look truncated into nothingness.
Always display "big" handles, due to the numerous complaints of the slim handles not being obvious

Test Plan:
Before:
{F7882628}
After:
{F7882624}

Reviewers: #plasma, #breeze, #vdg, ngraham, hpereiradacosta

Reviewed By: #vdg, ngraham, hpereiradacosta

Subscribers: ndavis, ahiemstra, hpereiradacosta, mthw, plasma-devel

Tags: #plasma

Maniphest Tasks: T9126

Differential Revision: https://phabricator.kde.org/D26655