[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: {F7542106}

Reviewers: romangg, #plasma, #vdg

Reviewed By: romangg, #plasma

Subscribers: GB_2, davidedmundson, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24479