Add an Activities list into the context menu

Summary:
This makes the task manager's context menu match KWin's titlebar context
menu.

BUG: 261649
CCBUG: 360293

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1815