[plasma-active-window-control] Use new activities icon

Summary:
Use the new icon added in D22155 and use an alternative icon for the "Buttons" category.

Test Plan: Open the context menu of the active window control widget and also open the widget settings.

Reviewers: Plasma, VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22169