Add tooltip to configure panel button

Summary:
Adds a tooltip to the configure panel button.

Test Plan: Hover over the configure panel button

Reviewers: Plasma, VDG, ndavis, ngraham

Reviewed By: VDG, ndavis, ngraham

Subscribers: Codezela, ngraham, ndavis, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18381