even slimmer scrollbars

Summary:
the scrollbar handle will be very slim normally, and grow a bit on mouse
over, this makes it work better with QtQuickControls2 listviews where
the contents overlap the scrollbar (as other platform are going in
this direction since they all have scrollbars visible only on scroll
or mouse over)
this makes scrollbars appear more light but still always visible to
convey information

right now follows animations enabled to use this, as the scrollbar groove already did that, tough a "slim scrollbars" settings may make sense

Test Plan: works both as qwidget and with the qstyle-based qqc2-desktop-style

Reviewers: #plasma, #kirigami, #vdg, hpereiradacosta, abetts

Reviewed By: #plasma, #vdg, hpereiradacosta, abetts

Subscribers: davidedmundson, ngraham, colomar, abetts, plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D9792