fix behavior of scrollable systray popups

Summary:
it's very rare that the icon list in the hidden area
of the systray is long enough to get a scrollbar, but
when it does it had a couple of issues:
sometimes it gets an horizontal scrollbar too and
the wheel events gets eaten:
always disable the horizontal scrollbar and
ignore wheel events over the label.
wheel events over the icon are still managed by
the applet (so still possible to for instance
adjusting the volume on pa applet)

BUG:372050

Test Plan:
hidden all the cions to enable scrollbar: wheel over
the text scrolls the view, over the icon adjusts the volume

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3263