make scrollbar size configurable

Summary:
make scrollbars size configurable with an enum, letting 3
allowed values, small, medium, large preferring it over
letting the user fiddle with pixel values
the default now is a tiny scrollbar.
(that can be done instead if needed)

Test Plan:
tried the 3 different values and checked that it updates
on the fly

Reviewers: #plasma, #vdg, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: colomar, alex-l, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3210