Add alternatives button to applet configuration in panel edit mode

Summary:
There is a configure button and an alternatives button in the context menu of an applet, but there is no alternatives button in the popup/overlay of an applet in the panel edit mode, so this adds that button, to make it more consistent and easier to find.

BUG: 405082
FIXED-IN: 5.16.0

Test Plan: Hover over an applet in the panel edit mode.

Reviewers: Plasma, VDG, davidedmundson, broulik, ngraham

Reviewed By: Plasma, VDG, broulik, ngraham

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

Tags: Plasma, VDG

Maniphest Tasks: T10047

Differential Revision: https://phabricator.kde.org/D17410