Use new toolbox icon for "Tools" dropdown menu button

Summary: Icon will be available in Frameworks 5.55. This repo uses the Plasma release schedule, so we can use it since Plasma 5.16 will probably be able to depend on Frameworks 5.55.

Test Plan:

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18395