manage scrollwheel by hand on desktop

Summary:
when on desktop, skip the flickable internal
scrollwheel manah=gement that is completely
broken and manage scrolling by ourselves
with a MouseArea instead
BUG:376910

Test Plan:
tried mouse wheel and touchpad two finger
scrolling, both look more sensible now
(how many lines the mouse wheel scrolls
is still to be implemented)
exported QT_QUICK_CONTROLS_MOBILE=true
env var and it works again as before

Reviewers: #kirigami

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D5004