[Panel] Use open/closed grabby hand to communicate that panel widgets are draggable

Summary:
Improve the visual hints that widgets are draggable while in panel edit mode. Needed to
bump the QtQuick import to make mouseArea.containsPress available.

Test Plan:

Reviewers: VDG, Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25998