[KCM] Make multi-screen draggability more obvious
Summary:
Add a label and use a grabby hand cursor when hovering over a screen when there are
more than one.
BUG: 412303
FIXED-IN: 5.18.0
Test Plan:
Reviewers: romangg, Plasma, VDG
Reviewed By: romangg, Plasma
Subscribers: GB_2, davidedmundson, broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24479