Change panel edit mode icon from from a hamburger icon to a configure icon

Summary:
Changes the panel edit mode icon from a hamburger icon to a configure icon (as suggested in this Phabricator task: https://phabricator.kde.org/T10047).

Test Plan: Look at the panel.

Reviewers: Plasma, VDG, ngraham, davidedmundson

Reviewed By: Plasma, VDG, ngraham, davidedmundson

Subscribers: davidedmundson, abetts, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma, VDG

Maniphest Tasks: T10047

Differential Revision: https://phabricator.kde.org/D17321